The 500 page was not loading theme CSS, so always appeared in light
theme. This extracts the stylesheets into a shared template used by all
pages.
<img width="1262" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/72222b8d-a523-40d0-aa91-b9db32e5625d">
Small related rant: I think there should only be one `<head>` in all of
the templates, but I see it was deliberatly done that the 500 page has
its own `<head>` because "it should only depend the minimal template
functions", but I disagree because we are missing a lot of things that
are in the regular `<head>`.
