Fix invalid rgba function usage in navigation.styl

[aaronedev]

This patch addresses a bug where rgba() was incorrectly being passed CSS variables resolving to hex codes, instead of using standard RGB triplets or hardcoded hex strings, producing invalid CSS. The patch replaces the offending usages with direct hex code strings as requested. It also addresses the inadvertent overriding of var(--theme-dark) in the process.

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PR created automatically by Jules for task 3359112623689405120 started by @aaronedev

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