stylix: set GTK icon theme

[laycookie]

Closes: #458

[trueNAHO]

Is this PR ready for review? I will mark this PR as draft for organizational reasons. Feel free to mark this PR as ready when it is.

Make sure to address the GitHub warnings when looking at https://github.com/danth/stylix/pull/603/files.

[laycookie]

I resolved all the linting warnings, however I was unable to test this build because I broke my nix distro. (long story) It should still work but if anyone could test my branch that would be ideal.
Note: If you enable iconTheme, and set a package option, but not set light or dark which correspond to the names of the iconTheme for there appropriate polarity, the config will fail to build. This is intended behavior which was inherited directly from gtk.

If you just set name for one polarity, it will build and use that polarity icon theme for both polarities.

[trueNAHO]

Reviewed-by: NAHO <[email protected]>