Weird behaviour of editor in the examples tab

[aannleax]

When selecting the examples you get a preview of the nemo rules in a editor. Two problems with that:

• Text highlights are not visible
• When scrolling down the top line sticks, but has a transparent background, which overlaps with the text

[mmarx]

The sticky top line is probably due to the file-level comment (which is present in all examples) that is also sticky (but with a non-transparent background) in the main editor

[aannleax]

yeah it seems like the editor as a whole has a transparent background, which causes this