Markdown Editor: Mermaid rendering doesn't use dark theme

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version:
>Version: 1.134.0-insider (user setup)
Commit: f844934f854c9277a48627aac77ebcea69591ae1
Date: 2026-08-11T22:56:11Z
Electron: 42.8.0
ElectronBuildId: 14845705
Chromium: 148.0.7778.280
Node.js: 24.18.0
V8: 14.8.178.38-electron.0
@github/copilot: 1.0.79-9
@github/copilot-sdk: 1.0.10-preview.0
OS: f844934f854c9277a48627aac77ebcea69591ae1
- OS Version:
>Edition Windows 11 Enterprise
Version 24H2
OS build 26100.8875
Experience Windows Feature Experience Pack 1000.26100.334.0

Steps to Reproduce:

1. Create `mermaid` diagram within a `md` file
2. Use `Markdown Editor` to render `md` file

Expected:
Mermaid blocks are rendered in dark theme (like in `Markdown Preview`).

Actual:
Mermaid blocks are rendered in light theme.