developer: Document CSS variables and classes for public use

[susnux]

☑️ Resolves

Currently you have to use try-and-error or read our source code for getting the CSS variables you probably want to use, so we should have one place to document this. As there is nothing yet lets start here. Also remove outdated Nextcloud 25 theming and replace with current CSS variables.

Added a go.php link so we can link to this docs from the nextcloud vue component documentations (and fixed 2 other links that do not exist anymore).

🖼️ Screenshots