`font` option under `theme`

### Description

Currently, if you want to modify the font, you must use the `custom-css-file` option.

It would be more convenient to have a dedicated `font` option, in my opinion, especially if you prefer a single-file configuration.

Perhaps this feature could handle font importing automatically via utilising [Google Fonts](https://fonts.google.com)/[Bunny Fonts](https://fonts.bunny.net).

The issue mentions the theme configuration and existing custom-css-file option, but names no files, tests, or entry points. Start by locating those configuration paths; done should mean a dedicated theme.font option with the expected font-import behavior, after the supported provider and fallback behavior are agreed.

Written by the indexing model from the issue text.