rust: integration with an existing colours crate

[backwardspy]

one option as suggested on discord: https://crates.io/crates/css-colors

it would be nice to either use these colour types directly, or provide a feature flag gated integration with the crate. that way we won't need to duplicate all its functionality for our own custom colour type.

[nyxkrage]

And in case we do proceeds with using css-colors types directly, it would be wise to open an issue in css-colors about integrating with palette another rust colors crate, which has a lot more functionality, but lacks the option of formatting directly as css values, which imo feels like a pretty solid feature for this crate.
Palette is used in a lot of larger Rust projects, so it would make sense to be able to convert between our color types, whether that will be our own, or css-colors, and palette's