💡 Allow setting custom fonts

⚬ PROBLEM: The font changer requested on #209 (located at General > More > Font on the UI) lets you choose a font among a fixed list (Comfortaa, Lato, Merriweather...).

⚬ SOLUTION: Allow the user to specify a custom font by typing its name.

First solution that came to mind: add a "Other..." option on the checklist, which, if selected, shows an input box below the "Font" option, maybe with a placeholder like "Liberation Sans"

⚬ ALTERNATIVES:

• Overriding CSS using other extensions like Stylish

⚬ RELEVANCE / SCOPE: I think that, particularly, users interested in ricing their web experience would love this feature! :-)

Also, it would indirectly solve feature requests for font introducing, like #2793, #2912 and so on.

⚬ "SIDE EFFECTS": It would make the "Settings" and "Font" lists a little longer, but only if the user chooses to.

⚬ CONTEXT: Users should verify that the font is accessible from their environment, whether they defined a built-in font, or a font locally installed by them.
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Thank you!

Start by tracing the existing font changer at Settings > General > More > Font and review the implementation described in issue #209. Check how the current fixed font list is stored and rendered, then verify that a user-entered font is applied when it is available locally and that the custom option behaves correctly in the settings UI.