Fix theme-auto loading (#23504)

Fix regression from https://github.com/go-gitea/gitea/pull/23481.

The conditional on the CSS import was being stripped away by webpack's
`css-loader`, resulting in the dark theme always loading. The old syntax
with `@import` nested inside `@media` also did not work as `css-loader`
(rightfully) ignores such non-standard `@import` syntax that was
previously supported by Less.

Unfortunately, we have to re-introduce postcss to the CSS pipeline to
fix this and I loaded only the minimal plugins to make it work.

There is one variant of the fix that does work without postcss, which is
to exclude the file from transpilation but I did not consider it as it
would have meant the `@import` was being done without a version suffix
in the URL, which would have caused cache issue.

Related: https://github.com/webpack-contrib/css-loader/issues/1503

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

web_src/css/themes/theme-auto.css

@@ -1 +1,2 @@
+/* This import requires postcss-import so the media query is preserved in the output */
 @import "./theme-arc-green.css" (prefers-color-scheme: dark);