Themes Syntax

Mermaid Diagrams

A comprehensive guide to creating diagrams with Mermaid in Hugo

This theme supports Mermaid diagrams directly in your Markdown content. Mermaid lets you create diagrams and visualizations using text and code.

About Mermaid.js

This theme integrates Mermaid.js (v11) to render diagrams from text definitions within Markdown code blocks. Mermaid is a JavaScript-based diagramming and charting tool that uses text-based syntax inspired by Markdown.

For complete syntax documentation, see the Mermaid.js documentation.

Getting Started

To create a Mermaid diagram, simply use a fenced code block with mermaid as the language identifier:

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```mermaid
graph TD
    A[Start] --> B[Process]
    B --> C[End]
```

The diagram will be automatically rendered when the page loads.

Features

  • Auto-detection: Mermaid script only loads on pages that contain diagrams
  • Theme Support: Diagrams automatically adapt to light/dark mode
  • HTML Labels: Support for HTML content in labels (like <br/> for line breaks)
  • Configurable: Customize version, security level, and more in your site config

Configuration

You can configure Mermaid in your site config:

hugo.yaml:

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params:
  article:
    mermaid:
      version: "11"           # Mermaid version from CDN
      look: classic           # classic or handDrawn (sketch style)
      lightTheme: default     # Theme for light mode
      darkTheme: neutral      # Theme for dark mode
      securityLevel: strict   # strict (default), loose, antiscript, sandbox
      htmlLabels: true        # Enable HTML in labels

hugo.toml:

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[params.article.mermaid]
  version = "11"           # Mermaid version from CDN
  look = "classic"         # classic or handDrawn (sketch style)
  lightTheme = "default"   # Theme for light mode
  darkTheme = "neutral"    # Theme for dark mode
  securityLevel = "strict" # strict (default), loose, antiscript, sandbox
  htmlLabels = true        # Enable HTML in labels

Additional Global Options

These optional settings use Mermaid’s defaults when not specified:

hugo.yaml:

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params:
  article:
    mermaid:
      maxTextSize: 50000      # Maximum text size (default: 50000)
      maxEdges: 500           # Maximum edges allowed (default: 500)
      fontSize: 16            # Global font size in pixels (default: 16)
      fontFamily: "arial"     # Global font family
      curve: "basis"          # Line curve: basis, cardinal, linear (default: basis)
      logLevel: 5             # Debug level 0-5, 0=debug, 5=fatal (default: 5)

hugo.toml:

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[params.article.mermaid]
  maxTextSize = 50000      # Maximum text size (default: 50000)
  maxEdges = 500           # Maximum edges allowed (default: 500)
  fontSize = 16            # Global font size in pixels (default: 16)
  fontFamily = "arial"     # Global font family
  curve = "basis"          # Line curve: basis, cardinal, linear (default: basis)
  logLevel = 5             # Debug level 0-5, 0=debug, 5=fatal (default: 5)

For diagram-specific options (like flowchart.useMaxWidth), use Mermaid’s init directive directly in your diagram:

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```mermaid
%%{init: {'flowchart': {'useMaxWidth': false}}}%%
flowchart LR
    A --> B
```

Security Note: The default securityLevel: strict is recommended. Set to loose only if you need HTML labels like <br/> in your diagrams.

Available Themes

Theme Description
default Standard colorful theme
neutral Grayscale, great for printing and dark mode
dark Designed for dark backgrounds
forest Green color palette
base Minimal theme, customizable with themeVariables
null Disable theming entirely

Custom Theme Variables

For full control, use the base theme with custom variables:

hugo.yaml:

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params:
  article:
    mermaid:
      lightTheme: base
      darkTheme: base
      lightThemeVariables:
        primaryColor: "#4a90d9"
        primaryTextColor: "#ffffff"
        lineColor: "#333333"
      darkThemeVariables:
        primaryColor: "#6ab0f3"
        primaryTextColor: "#ffffff"
        lineColor: "#cccccc"
        background: "#1a1a2e"

hugo.toml:

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[params.article.mermaid]
  lightTheme = "base"
  darkTheme = "base"

  [params.article.mermaid.lightThemeVariables]
    primaryColor = "#4a90d9"
    primaryTextColor = "#ffffff"
    lineColor = "#333333"

  [params.article.mermaid.darkThemeVariables]
    primaryColor = "#6ab0f3"
    primaryTextColor = "#ffffff"
    lineColor = "#cccccc"
    background = "#1a1a2e"

Common variables: primaryColor, secondaryColor, tertiaryColor, primaryTextColor, lineColor, background, fontFamily

Note: Theme variables only work with the base theme and must use hex color values (e.g., #ff0000).

Diagram Types

Flowchart

Flowcharts are the most common diagram type. Use graph or flowchart with direction indicators:

  • TD or TB: Top to bottom
  • BT: Bottom to top
  • LR: Left to right
  • RL: Right to left
  flowchart LR
    A[Hard edge] -->|Link text| B(Round edge)
    B --> C{Decision}
    C -->|One| D[Result one]
    C -->|Two| E[Result two]

Sequence Diagram

Perfect for showing interactions between components:

  sequenceDiagram
    participant Alice
    participant Bob
    Alice->>John: Hello John, how are you?
    loop Healthcheck
        John->>John: Fight against hypochondria
    end
    Note right of John: Rational thoughts <br/>prevail!
    John-->>Alice: Great!
    John->>Bob: How about you?
    Bob-->>John: Jolly good!

Class Diagram

Visualize class structures and relationships:

  classDiagram
    Animal <|-- Duck
    Animal <|-- Fish
    Animal <|-- Zebra
    Animal : +int age
    Animal : +String gender
    Animal: +isMammal()
    Animal: +mate()
    class Duck{
        +String beakColor
        +swim()
        +quack()
    }
    class Fish{
        -int sizeInFeet
        -canEat()
    }
    class Zebra{
        +bool is_wild
        +run()
    }

State Diagram

Model state machines and transitions:

  stateDiagram-v2
    [*] --> Still
    Still --> [*]

    Still --> Moving
    Moving --> Still
    Moving --> Crash
    Crash --> [*]

Entity Relationship Diagram

Document database schemas:

  erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    CUSTOMER }|..|{ DELIVERY-ADDRESS : uses
    CUSTOMER {
        string name
        string custNumber
        string sector
    }
    ORDER {
        int orderNumber
        string deliveryAddress
    }

Gantt Chart

Plan and track project schedules:

  gantt
    title A Gantt Diagram
    dateFormat YYYY-MM-DD
    section Section
        A task          :a1, 2024-01-01, 30d
        Another task    :after a1, 20d
    section Another
        Task in Another :2024-01-12, 12d
        another task    :24d

Pie Chart

Display proportional data:

  pie showData
    title Key elements in Product X
    "Calcium" : 42.96
    "Potassium" : 50.05
    "Magnesium" : 10.01
    "Iron" :  5

Git Graph

Visualize Git branching strategies:

  gitGraph
    commit
    commit
    branch develop
    checkout develop
    commit
    commit
    checkout main
    merge develop
    commit
    commit

Mindmap

Create hierarchical mindmaps:

  mindmap
  root((mindmap))
    Origins
      Long history
      Popularisation
        British popular psychology author Tony Buzan
    Research
      On effectiveness<br/>and features
      On Automatic creation
        Uses
            Creative techniques
            Strategic planning
            Argument mapping
    Tools
      Pen and paper
      Mermaid

Timeline

Display chronological events:

  timeline
    title History of Social Media Platform
    2002 : LinkedIn
    2004 : Facebook
         : Google
    2005 : YouTube
    2006 : Twitter

Advanced Features

HTML in Labels

To use HTML in labels, you must set securityLevel: loose in your site config:

hugo.yaml:

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params:
  article:
    mermaid:
      securityLevel: loose
      htmlLabels: true

hugo.toml:

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[params.article.mermaid]
  securityLevel = "loose"
  htmlLabels = true

Then you can use HTML tags like <br/> for line breaks:

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```mermaid
graph TD
    A[Line 1<br/>Line 2] --> B[<b>Bold</b> text]
```

Per-Diagram Theming

Override the theme for a specific diagram using Mermaid’s frontmatter:

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```mermaid
%%{init: {'theme': 'forest'}}%%
graph TD
    A[Start] --> B[End]
```

  %%{init: {'theme': 'forest'}}%%
graph TD
    A[Christmas] -->|Get money| B(Go shopping)
    B --> C{Let me think}
    C -->|One| D[Laptop]
    C -->|Two| E[iPhone]
    C -->|Three| F[Car]

Inline Styling with style

You can style individual nodes directly within your diagram using the style directive:

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```mermaid
flowchart LR
    A[Start] --> B[Process] --> C[End]
    style A fill:#4ade80,stroke:#166534,color:#000
    style B fill:#60a5fa,stroke:#1e40af,color:#000
    style C fill:#f87171,stroke:#991b1b,color:#fff
```

Result:

  flowchart LR
    A[Start] --> B[Process] --> C[End]
    style A fill:#4ade80,stroke:#166534,color:#000
    style B fill:#60a5fa,stroke:#1e40af,color:#000
    style C fill:#f87171,stroke:#991b1b,color:#fff

Style properties include:

  • fill - Background color
  • stroke - Border color
  • stroke-width - Border thickness
  • color - Text color
  • stroke-dasharray - Dashed border (e.g., 5 5)

Styling with CSS Classes

You can define reusable styles with classDef and apply them using :::className:

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```mermaid
flowchart LR
    A:::success --> B:::info --> C:::warning
    classDef success fill:#4ade80,stroke:#166534,color:#000
    classDef info fill:#60a5fa,stroke:#1e40af,color:#000
    classDef warning fill:#fbbf24,stroke:#92400e,color:#000
```

Result:

  flowchart LR
    A:::success --> B:::info --> C:::warning
    classDef success fill:#4ade80,stroke:#166534,color:#000
    classDef info fill:#60a5fa,stroke:#1e40af,color:#000
    classDef warning fill:#fbbf24,stroke:#92400e,color:#000

Subgraphs

Group related nodes together:

  flowchart TB
    subgraph one
        a1-->a2
    end
    subgraph two
        b1-->b2
    end
    subgraph three
        c1-->c2
    end
    one --> two
    three --> two
    two --> c2

Theme Switching

This theme automatically detects your site’s light/dark mode preference and adjusts the Mermaid diagram theme accordingly:

  • Light mode: Uses the default Mermaid theme
  • Dark mode: Uses the dark Mermaid theme (configurable)

Try toggling the theme switcher to see diagrams update in real-time!

Complex Example

Here’s an example with subgraphs, HTML labels, emojis, and custom styling:

  flowchart TD
  subgraph client["👤 Client"]
      A["User Device<br/>192.168.1.10"]
  end

  subgraph cloud["☁️ Cloud Gateway"]
      B["Load Balancer<br/>(SSL Termination)"]
  end

  subgraph server["🖥️ Application Server"]
      C["API Gateway<br/>10.0.0.1"]
      D["Auth Service<br/>10.0.0.2"]
      E["Web Server<br/>10.0.0.3"]
      F["Database<br/>10.0.0.4"]
  end

  A -- "HTTPS Request" --> B
  B -- "Forward<br/>(internal)" --> C
  C -- "Authenticate" --> D
  D -- "Token" --> C
  C -- "Route" --> E
  E --> F

  style client fill:#1a365d,stroke:#2c5282,color:#fff
  style cloud fill:#f6ad55,stroke:#dd6b20,color:#000
  style server fill:#276749,stroke:#22543d,color:#fff

Note: This example requires securityLevel: loose for HTML labels and styling to work.

Known Limitations

Dark Mode Theming

Mermaid.js’s built-in themes have some limitations:

  • dark theme (default): Best text contrast, but some diagram backgrounds may appear brownish (e.g., Gantt charts)
  • neutral theme: Better background colors, but some text (labels, legends) may have reduced contrast

For full control, use the base theme with custom variables:

hugo.yaml:

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params:
  article:
    mermaid:
      darkTheme: base
      darkThemeVariables:
        primaryColor: "#1f2937"
        primaryTextColor: "#ffffff"
        lineColor: "#9ca3af"
        textColor: "#e5e7eb"

hugo.toml:

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[params.article.mermaid]
  darkTheme = "base"

  [params.article.mermaid.darkThemeVariables]
    primaryColor = "#1f2937"
    primaryTextColor = "#ffffff"
    lineColor = "#9ca3af"
    textColor = "#e5e7eb"

We plan to improve dark mode theming in future updates as Mermaid.js evolves.

Troubleshooting

Diagram not rendering?

  1. Make sure you’re using a fenced code block with mermaid as the language
  2. Check your browser’s console for syntax errors
  3. Verify your Mermaid syntax at Mermaid Live Editor

HTML not working in labels?

HTML in labels requires securityLevel: loose. Update your configuration:

hugo.yaml:

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params:
  article:
    mermaid:
      securityLevel: loose
      htmlLabels: true

hugo.toml:

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[params.article.mermaid]
  securityLevel = "loose"
  htmlLabels = true

Warning: Using loose security level allows HTML in diagrams. Only use this if you trust your diagram content.

Syntax errors?

Mermaid is strict about syntax. Common issues:

  • Missing spaces around arrows
  • Unclosed brackets or quotes
  • Invalid node IDs (avoid special characters)

Resources

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