Well, setting #content pre { background: #2f0e0f; } to #content pre { background: transparent; } fixed it. Results in a white background, so I then set .CodeMirror { color: white; background: #2f0e0f; } to get what I wanted, but the selection is properly displayed now.
The #content pre change seems to only have visible consequences in the wiki editor with CM on.
<pre> is barely used anywhere else on the wiki and honestly I can live with transparent bg and just a thicker/colored border in those cases
Anyway, thanks again