Mermaid Diagram Skill

This skill provides guidance on creating beautiful, professional Mermaid diagrams that render correctly on GitHub and work well in both light and dark mode.

Core Principles

Use dark fills with light strokes — Ensures readability in both light and dark mode
Set subgraph fills to none — Allows subgraphs to adapt to any background
Use rounded shapes — ([text]) for stadium shapes, ((text)) for circles
No Font Awesome icons — GitHub doesn't support fa:fa-* icons, they render as text
Quote subgraph labels — Use subgraph Name["Label Text"] syntax
Define classDef styles at the top — Keep all styling together for maintainability

The Golden Rule: Dark Fills + Light Strokes

The key insight for dark/light mode compatibility:

classDef myStyle fill:#DARK_COLOUR,stroke:#LIGHT_COLOUR,stroke-width:2px,color:#fff

Fill: Use a darker shade (the node background)
Stroke: Use a lighter shade of the same colour family (the border)
Color: Always #fff (white text on dark background)

This approach ensures nodes are readable regardless of the page background.

GitHub-Compatible Template

This is the canonical template for GitHub-rendered Mermaid diagrams:

flowchart TD
    %% --- COLOUR PALETTE & STYLING ---
    %% Dark fills + light strokes = readable in both light and dark mode
    classDef user fill:#374151,stroke:#d1d5db,stroke-width:2px,color:#fff
    classDef primary fill:#5b21b6,stroke:#ddd6fe,stroke-width:2px,color:#fff
    classDef secondary fill:#1e40af,stroke:#bfdbfe,stroke-width:2px,color:#fff
    classDef accent fill:#c2410c,stroke:#fed7aa,stroke-width:2px,color:#fff
    classDef success fill:#047857,stroke:#a7f3d0,stroke-width:2px,color:#fff

    %% --- NODES ---
    User((User)):::user
    User --> Action(["Performs action"]):::user

    Action --> Primary

    subgraph Primary["Primary Component"]
        direction TB
        Step1(["Step 1"]):::primary
        Step2(["Step 2"]):::primary
        Step1 --> Step2
    end

    subgraph Secondary["Secondary Component"]
        direction TB
        Process(["Process"]):::secondary
    end

    Primary --> Secondary
    Secondary --> Output(["Output"]):::success

    %% --- SUBGRAPH STYLES ---
    %% fill:none allows subgraphs to adapt to any background
    style Primary fill:none,stroke:#8b5cf6,stroke-width:2px,stroke-dasharray:5 5,color:#8b5cf6
    style Secondary fill:none,stroke:#3b82f6,stroke-width:2px,color:#3b82f6

Colour Pairing Examples

Choose any colours you like — just follow the dark fill + light stroke pattern:

Fill (Dark)	Stroke (Light)	Result
`#374151`	`#d1d5db`	Grey
`#5b21b6`	`#ddd6fe`	Purple
`#1e40af`	`#bfdbfe`	Blue
`#c2410c`	`#fed7aa`	Orange
`#047857`	`#a7f3d0`	Green
`#b91c1c`	`#fecaca`	Red
`#0f766e`	`#99f6e4`	Teal

These are just examples. Use whatever colours suit your diagram — the principle is what matters.

Subgraph Syntax

Wrong — Causes parse error

subgraph MyGroup [Label With Spaces]

Correct — Quote the label

subgraph MyGroup["Label With Spaces"]

Node Shapes

Wrong — Square brackets are harsh

A[Square Node]

Correct — Use rounded shapes

A(["Stadium shape"])     %% Rounded ends - use for most nodes
B((Circle))              %% Circle - use for users/actors
C{{"Decision"}}          %% Hexagon for decisions
D[(Database)]            %% Cylinder for databases/storage

Subgraph Styling

Wrong — Coloured fills break in dark mode

style MySubgraph fill:#f0f9ff,stroke:#3182ce

Correct — Transparent fills adapt to any background

style MySubgraph fill:none,stroke:#8b5cf6,stroke-width:2px,stroke-dasharray:5 5,color:#8b5cf6

Key points:

fill:none makes the background transparent
stroke-dasharray:5 5 creates a dashed border (optional, looks clean)
color:#... sets the subgraph label colour to match the border

Link Styling

A --> B              %% Solid arrow
A -.-> B             %% Dashed arrow
A -.->|Label| B      %% Dashed arrow with label
A ==> B              %% Thick arrow

Complete Example

flowchart TD
    classDef user fill:#374151,stroke:#d1d5db,stroke-width:2px,color:#fff
    classDef process fill:#5b21b6,stroke:#ddd6fe,stroke-width:2px,color:#fff
    classDef decision fill:#c2410c,stroke:#fed7aa,stroke-width:2px,color:#fff
    classDef success fill:#047857,stroke:#a7f3d0,stroke-width:2px,color:#fff

    User((User)):::user
    User --> Request(["Makes request"]):::user

    Request --> Process

    subgraph Process["Processing"]
        direction TB
        Validate(["Validate input"]):::process
        Execute(["Execute logic"]):::process
        Validate --> Execute
    end

    Execute --> Check{{"Success?"}}:::decision
    Check -->|Yes| Done(["Complete"]):::success
    Check -->|No| Request

    style Process fill:none,stroke:#8b5cf6,stroke-width:2px,stroke-dasharray:5 5,color:#8b5cf6

Common Mistakes

Font Awesome icons (GitHub doesn't support them)

A[fa:fa-user User]  %% Renders as literal text

Light fills with dark text

classDef bad fill:#ffffff,stroke:#000000,color:#000000  %% Invisible in dark mode

Coloured subgraph fills

style Sub fill:#e0f2fe  %% Looks different in light vs dark mode

Unquoted subgraph labels with spaces

subgraph Sub [My Label]  %% Parse error!

Quick Reference

flowchart TD
    %% 1. Define styles: dark fill + light stroke + white text
    classDef myStyle fill:#DARK,stroke:#LIGHT,stroke-width:2px,color:#fff

    %% 2. Use rounded shapes
    Node(["Text"]):::myStyle

    %% 3. Quote subgraph labels
    subgraph Sub["My Label"]
        Inner(["Inner"])
    end

    %% 4. Style subgraphs with fill:none
    style Sub fill:none,stroke:#COLOR,stroke-width:2px,color:#COLOR

When to Use This Skill

Invoke this skill when creating:

Architecture diagrams for PRs
System flow documentation
Data pipeline visualisations
Process flowcharts
Any diagram in GitHub markdown

GitHub-Specific Notes

No Font Awesome — GitHub's Mermaid renderer doesn't support FA icons
No HTML — Can't use <br> or other HTML in node labels
Quote labels with spaces — subgraph X["Label"] not subgraph X [Label]
Test locally — Use mermaid.live to preview before committing

mermaid