Use semantic Tailwind color names so the admin can be themed

Desired Behavior

I would like to be able to theme the Solidus admin as I am currently able now with the legacy backend. There we have bootstrap variables that can be changed to new color values. The use of generic variables (primary, secondary, danger, brand, etc.) makes it very easy to change the look of Solidus admin. Also light and darkmode are easier to maintain that way.

Current Behavior

We currently have color based class names all over the new admin. This makes it very hard to change the look of the new admin. Also it makes it much harder for us to change the theme in the future and make adjustments to the light and dark modes.

Additional notes

Solidus is a framework that allows clients to adopt the admin to their needs. This includes theming. IMO this is one of reasons for the success of Solidus in the community. We should not remove this feature that many stores rely on.

In the tailwind docs are ways described to make this more maintainable for us. https://tailwindcss.com/docs/customizing-colors#naming-your-colors

I think we should move to a more framework-alike approach with the new admin.

Start with admin/config/tailwind.config.js and the Tailwind documentation on naming colors. Search the new admin for the color-based class names described in the issue and map the affected styling usage. Done means the admin uses semantic color names consistently and its theme, including light and dark modes, can be customized as described.