[x]):When I change the theme in /data/gitea/conf/app.ini, so that it reads:
...
[repository.upload]
TEMP_PATH = /data/gitea/uploads
[ui]
DEFAULT_THEME = arc-green
[server]
APP_DATA_PATH = /data/gitea
...
The theme will change on public facing pages, i.e., those before I've logged in... But as soon as I log in, everything returns to regular light/gitea theme.


You need to change it in your account settings, e.g. https://try.gitea.io/user/settings/account.
Each user can change their preferred theme.
Holy chips that was fast. Thanks. :)