1. The "close" inside "modal" are likely broken for long time
* There is no var called `--body-color`
* There is no `fullscreen modal`
* The `.ui.modal > .close.inside` doesn't seem to match most icons. It
only matches a few like "fork-repo-modal" or "adopt repo". Other places
are just buggy code copied again and again.
2. Convert the legacy `&:hover` LESS syntax to CSS syntax
Ran most of the Less files through the Less compiler and Prettier and
then followed up with a round of manual fixes.
The Less compiler had unfortunately stripped all `//` style comments
that I had to restore (It did preserve `/* */` comments). Other fixes
include duplicate selector removal which were revealed after the
transpilation and which weren't caught by stylelint before but now are.
Fixes: https://github.com/go-gitea/gitea/issues/15565
