Request: add more colour options for themes

[Dehelssey]

Currently we have a few pre-selected options for theming Elk.
It'd be nice to have more of them, either with a color picker or an hex field 🤗

[shuuji3]

I saw a similar suggestion in the past (#1365), but it has been stalled for a while.

I feel like it's fine to add a color picker so that users can pick up their favorite color, which could make them a bit more pleasant using Elk. I think adding an option won't prevent using the currently recommended palettes.

Optionally, we could add a warning if the color contrast is weak to help choose a good color for better readability.