[Accessibility] Weak contrast when selecting tags

[codebude]

Quoted from @DawidGardzinski in the original issue Feedback after 1-2 weeks of using Libris (#78).

The selection indicator is barely visible on some screens. It may depend on the theme, but it would be good to keep an eye on this - ideally by checking the contrast, or at least by using bold text.

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Author: @DawidGardzinski
Original issue: #78