spicetify: allow other themes to be used alongside stylix

[dastarruer]

Basically, spicetify-nix allows you to set a custom color scheme. So if we could use our own theme, and substitute the colors with stylix, that would be great.

[danth]

Some general advice- in your config you can write ${config.lib.stylix.colors.base00} to access the current Stylix theme. I'm not familiar with spicetify in particular, but it should be possible to disable the built in target and set the spicetify options directly using your own theme based on config.lib.stylix.colors.