Generalize theming

[TylerBloom]

Currently, the gruvbox hard dark theme is hard coded into everything (including the syntax highlighting). I would like to make this more extensible, potentially going so far as to allow for customizability for users.

[TylerBloom]

This has been partially addressed. When the Yew + Ratatui integrations were extracted, I created the base16-palettes crate. This allows the backend renderer to render in different themes based on what palette is loaded in the renderer without needing to adjust the widget logic. This should make adding this feature relatively easy.