Instance custom CSS

Currently, customization of selfhosted instances is very limited. Fluxer already encourages deeper customization with CSS, but it's currently only editable by the user and not accessible to the admin to change the default styling.

The admin panel should have a setting textarea for custom CSS. It could either:

• Append the CSS in addition to the user CSS system. User CSS coexists and can override it e.g. with !important. Maybe control precedence through @layer?
• Make use of the existing user CSS setting by making the instance CSS be the default. This makes it easy to tweak by the user, but updates to the CSS cannot apply to users in a dirty state.

This is specifically about applying CSS to the web frontend served by the instance. This is different from e.g.:

• Instance applying CSS to the desktop client
• Guild applying CSS to the web frontend or desktop client

Because there's no issue of trust - the same server that is serving the frontend simply needs to append the admin defined custom CSS, I think these would be useful too but they present much more complex UX and security concerns.