fix(ui): apply background color to wiki content (#9400)

Applying a distinct background color to wiki content ensures that content with a background color (eg. code blocks) actually stands out against the background.

Fixes #9391.

Before:
- https://codeberg.org/forgejo/forgejo/attachments/72c2530b-a79b-45e7-8980-db0b33a4ca85
- https://codeberg.org/forgejo/forgejo/attachments/3195cc83-1f8b-46c8-8eb8-2b1ceb0bce28
- https://codeberg.org/forgejo/forgejo/attachments/8922db5a-c57b-44e5-a4de-9cfbd0ba207f
- https://codeberg.org/forgejo/forgejo/attachments/5815022f-f03a-476b-9dec-b224c4c14f12

After:
- https://codeberg.org/forgejo/forgejo/attachments/f3a0cb17-b829-4bc6-ae07-f06d111a103e
- https://codeberg.org/forgejo/forgejo/attachments/e1cf97e5-44a0-4dce-85ab-a594c4474ab8 (same)
- https://codeberg.org/forgejo/forgejo/attachments/7edd5163-186c-488a-a6c1-b093edc0be43
- https://codeberg.org/forgejo/forgejo/attachments/5815022f-f03a-476b-9dec-b224c4c14f12 (same)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9400
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Jordan Atwood <nightfirecat@nightfirec.at>
Co-committed-by: Jordan Atwood <nightfirecat@nightfirec.at>

web_src/css/repo/wiki.css

@@ -24,6 +24,7 @@
 }
 
 .repository.wiki .wiki-content-parts .markup {
+  background-color: var(--color-box-body);
   border: 1px solid var(--color-secondary);
   border-radius: var(--border-radius);
   padding: 1em;