Instead of manually just setting faces after loading a doom-themes
theme, let's use a custom override theme which we apply right after
applying any doom-themes theme.
The override theme uses various doom-color helpers, so the colors it
uses will be based on the most recently applied doom-themes theme.
Change style back to a plain vertical line, but slightly narrower than
last time which I ended up not liking, while also aligning it to the
right hand side of the left fringe, so there's a gap between the window
border and the diff-hl status line. Also tweak the colors a bit.
I feel this works better, and keeps things nice and clean.
Due to changes to use-package's :custom option, my abuse of it to set
custom variables no longer works and yields errors.
Instead correctly set custom variables with defvar.
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.
I've been using doom-themes' doom-vibrant for long enough now that I'm
certain I won't be moving away from it anytime soon. So let's simplify
and strip away all other theme setup stuff.
