Allow UI customization

[flaksp]

Because of Camellia uses CSS variables, it's possible to implement some customization functionality: change colors, sizes, etc (almost everything that exposed via CSS variables).

Customization should not allow inserting arbitrary CSS chunks, because some updates with breaking changes may naturally break the extension for some folks. We should create some kind of configurator like Telegram Desktop theme editor with list of allowed properties to modify. On top of that it is also possible to add ability to import and export themes.