GTK4 support

[Zipdox2]

 * All themes
 * Cinnamon 5.6.8
 * Debian sid
 * AMD Radeon RX 470 (mesa 22.3.6-1)
 * Snippet or attachment of ~/.cinnamon/glass.log (doesn't exist)
 * Snippet or attachment of ~/.xsession-errors (doesn't have relevant errors)

Notify author of theme
@smurphos
@germanfr
@frombenny
@feren
@brownsr
@Cloweling
@Vanov
@Bundy01
@andr35
@Elbullazul
@getkey
@praveenp
@Anime-is-real
@bomdia
@i-vrnv
@freemangilgamesh
@Radek71
@lleeoochen
@rajat

Issue
GTK4 apps aren't themed because there's no gtk-4.0 folder in any theme. GTK4 apps run with light mode and the window contols don't match the rest of the system.

Steps to reproduce
Run a GTK4 app like gnome-calculator.

Expected behavior
App adopts the selected theme.

Other information
I tried simply copy-pasting the gtk-3.0 folder in CBlack and naming it gtk-4.0. This did work, but every widget had an annoying dotted border. I commented out line 12 of gtk.css (outline-style: dashed;) which fixed it, but the padding of widgets is off. Corners also aren't rounded. Additionally, GTK is throwing a bunch of warning theme parser errors.

[Elbullazul]

The Windows 10 theme has support for GTK4, I'll open a pull request to merge it in the coming days

[Bundy01]

The CBlue and CGreen themes are only effective with the cinnamon appearance.
I haven't created any gtk themes. Check which gtk theme you're using and tell them about your problem.

I'm using the 'vimix-dark-compact-doder' theme and I'm also experiencing this inconvenience.
Regards.