Mermaid .mmd file previews are low contrast and illegible in Dark Mode

[sfriedman-cape]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Code Search and Navigation

Body

I'm trying to view the preview for a sequence diagram in an .mmd file, similar to:

sequenceDiagram
    autonumber

    participant Bob
    participant Alice
    participant Tim

    Bob->>Bob: Bob takes Action A
    Bob->>Bob: Bob takes Action B

    Bob->>Alice: Give a key to Alice
    Alice->>Bob: Return a response back to Bob

    Bob->>Tim: Give a key to Tim
    Tim->>Bob: Success

The rendering for the file in Dark Mode is illegible. Each of the actions is rendered as gray text on a black background, which is very difficult to read.

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[queenofcorgis]

Hi @sfriedman-cape 👋🏼 ,

Thanks for reporting this, to help our team best investigate would you mind sharing your operating system, browser and version, and any screenshots/recordings of the preview please?

[KhyFee]

+1 as an accessibility / dark-mode contrast issue, not only cosmetics.

Gray text on near-black Mermaid sequence steps fails basic text contrast and makes .mmd previews unusable in Dark Mode. The renderer should either:

1. ship dark-aware Mermaid theme tokens, or
2. force a light diagram canvas with dark text so contrast never depends on the outer GitHub theme.

Temporary docs workaround: export SVG/PNG with an explicit light theme for critical diagrams. Still worth fixing native preview for reviewers reading in Dark Mode.