Consider storing UI colors in an asset for hot reloading

[MiniaczQ]

Mentioned in #16

[benfrankel]

Waiting on BSN, I suppose. I do have an implementation of this in my own repo fwiw:

• Config: https://github.com/benfrankel/bevy_jam_template/blob/main/src/util/config.rs
• Color config / hot reloading: https://github.com/benfrankel/bevy_jam_template/blob/main/src/core/theme.rs

[MiniaczQ]

Out of scope for 0.14 template like #71