[Meta] Clean up CSS within Admin theme

default_admin's CSS is a huge mess. IMO this should have been completed before it was merged into core. There will need to be many, many core commits (which introduce many delays).

• Gin has 41 Sass partials in Gin's styles/base directory that get compiled into one file. This one file was committed to default_admin. It's > 5660 lines of CSS that needs to be untangled.
• There's tons of shims for contrib modules (e.g. dropzone, coffee, layout paragraphs). How do we handle these?
• There's a number of Sass functions, mixins, and variables that need to be extracted in Gin's styles/helpers/
• The CSS in general is very old. Lots of comments regarding Drupal 9 and 10.
• It appears that the entirety of Claro's CSS was copied over. This is because Gin used Claro as a base theme.
• All in all, there's somewhere around 2-3 CSS files/partials per component. We need to figure this out.

On the good news front, I created https://github.com/mherchel/ddev-drupal-admin-vrt, which is a DDEV add-on that sets up a visual-regression system, so we can easily test for regressions.

i just found this issue. looking at the issue summary and mikes comment, would it be preferable to wait with opening more issues until this issue is in? and that most of those bugs are potentially obsolete after cleanup? found #3576874: Toggle switches are visually broken within dialog modals a few weeks ago but also have a hand full of bugs in ios that would need issues as well.

There's tons of shims for contrib modules (e.g. dropzone, coffee, layout paragraphs). How do we handle these?

These should all be removed. We already removed support for forum, book etc. #3581831: Remove support for Book and Forum Module.

would it be preferable to wait with opening more issues until this issue is in?

I say yeah. Even if the issues persist (which they may), they'll be blocked until we can get the CSS in a maintainable state.

I'll postpone CSS-related accessibility issues based on this so that would-be contributors don't waste time on them.

Regarding

5. Why is default admin theme changing Navigation toolbar styles? Should it be? If so, move those styles to Navigation module

Gonna be a similar issue with contextual_links.css, I think.

AFAICT, both of them currently affect dark mode by way of custom properties defined in variables.css.

Disregard previous comment about contextual_links.css. I conflated problems.

Another issue completed.

Tweaked "5. Remove any gin namespacing from CSS classes and variables" in the IS to include CSS selectors in PHP and template files.

Not sure where to report this. Figured y'all would know what to do.

Looks like like a theme hook is expecting a CSS file in the migration directory:

$base_css = $this->themeExtensionList->getPath('default_admin') . '/migration/css/base/gin.css';