Previously I've always kinda "hacked" things to get C-z prefixes for
various workspace-related packages to work. Now it's finally setup
correctly with a custom siren-workspace-map keymap bound to C-z, and
relevant packages adding keybindings to it.
And also improve zoom-window color selection by adding a doom-themes
override for zoom-window, removing the hard-coded modeline background
color which was used before.