Use CSS Variables for fonts, remove postcss-loader (#13204)

* Use CSS Variables for fonts, remove postcss-loader

- Use CSS variables for fonts, making the fonts easier to customize
- Remove postcss-loader, it's not doing anything useful and is actually
  applying strange transforms on our CSS.

Fixes: https://github.com/go-gitea/gitea/issues/11045

* introduce helper variable, mark documented vars

* work around case issue by always quoting specific fonts

package.json

@@ -32,9 +32,6 @@
     "mini-css-extract-plugin": "0.11.1",
     "monaco-editor": "0.20.0",
     "monaco-editor-webpack-plugin": "1.9.0",
-    "postcss": "7.0.32",
-    "postcss-loader": "4.0.1",
-    "postcss-preset-env": "6.7.0",
     "raw-loader": "4.0.1",
     "sortablejs": "1.10.2",
     "swagger-ui": "3.32.5",