grub: only change colors, not override the default nixos theme

[i-am-logger]

this is in regard to : https://github.com/danth/stylix/blob/679a80676840185cbc72a3751ccdac169477c21a/modules/grub/nixos.nix#L45

the expected behavior is to over the grum theme colors overwritten by stylix. currently stylix changes the entire theme.

current default stylix behavior:

when targed.grub.enable = false; the default nixos theme is on..

[mightyiam]

Not entirely related but if you care about GRUB you may care about the fonts it uses: #1206