Avoid styling headers like buttons

Originally created by @sajolida on #15114 (Redmine)

See attachment. “News” and “Security” are headers while “Windows” and “macOS” are buttons.

One way would be to use a consistent color on buttons (maybe green like we do for links).
Another way would be to avoid a plain dark background.

Feature Branch: feature/15114-remove-purple-rectangles

Parent Task: #14548

Edited May 15, 2020 by sajolida

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