Better highlight code blocks: background color

From my point of view, it would better highlight the code blocks by using a darker background.

assigned to @s9165289--tu-dresden.de

added Layout label

created branch 210-better-highlight-code-blocks-background-color to address this issue

mentioned in merge request !455 (merged)

changed milestone to %Meeting 2022-01-12

mentioned in commit a5b551f8

closed with merge request !455 (merged)