Move stylesheets to Granite

[wpkelso]

This is meant to be an overview/tracking/planning issue. Individual parts should be attached as sub-issues to this issue.

Rationale

1. Pre-empting the move to platform-handled styling in GTK 5
2. We probably shouldn't be overriding app CSS with a system stylesheet (Please don't theme our apps)
3. Apps built for elementary OS will likely look pretty broken on other operating systems, given that we currently don't have active support for GTK widgets in the Granite stylesheet

Current & Relevant Work

#703

elementary/stylesheet#1278

Current State

• granite:main as live (GTK stylesheet excluded)

• granite:main with GTK stylesheet included