Avoid styling headers like buttons

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

Related issues

• Related to #15597 (closed)
• Blocks #15392 (closed)

Original created by @sajolida on 15114 (Redmine)