[Fix] Mermaid labels are unreadable in light IDE themes (#1285)

* fix(webview): adapt Mermaid diagrams to IDE themes

* fix(webview): add Mermaid visual regression baselines for all IDE themes

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Co-authored-by: Roomote <roomote@roomote.dev>
Co-authored-by: Elliott de Launay <edelaunay@wealthsimple.com>