The most important changes here are fixes for the broken UI (dialogs/modals, OSDs) in cinnamon 6.4.

Additional changes:

• Merge a few commits in the original Adapta repo that weren't present here
• Convert some changes that were made directly to the compiled CSS here, back into the SCSS sources with mixins/color variables etc
• Build and include the stylesheet for GTK4. This is not quite as polished as the GTK3 one (certain GTK4 widgets should be improved), but it's functional
• Migrate the autoconf scripts and the SCSS sources from the deprecated sassc to dart sass (changes merged from this fork of Adapta by senatoreg).

The updated sass compiler is the reason for the huge diff. Anyway, the new generated CSS no longer puts every rule on a single line, so future diffs should be better than before (it also generates a source map). To see the actual changes to the source, check out the latest commits from me here. The changes for 6.4 are mostly in this commit.

(also ping @smurphos, though from what I can see he's been pretty inactive on github)

Fixes #842
Relevant to #749