fix issue 21273 - [spec] Inexistent contrast for shell snippets make …

[aG0aep6G]

…them unreadable (CSS)

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21273	major	[spec] Inexistent contrast for shell snippets make them unreadable (CSS)

[kinke]

Thanks!