Finish cleaning up CSS from move to CSS variables for colors

[sandymcfadden]

Fix

This is a continuation of moving to CSS variables for colors within the app.
This removes the remaining bits of theme classes and CSS code throughout the app.
This then required a bit of cleanup to fix some of the colors.

A couple of notes:

• The highlight.js CSS used to style markdown previews is the only remaining thing that does a specific check for light and dark mode to then load the appropriate file in. I'm not sure of a better way to do this.

• After the cleanup, there are a few minor differences in dark mode. Are we OK with these?

The color of the highlighted icon in the navigation bar from white to blue

Before

After

The color of the drag icon when editing tags is slightly different

Before

After

The color of delete forever and empty trash button when viewing trash

Before

After

Test

• Smoke test the app in both light and dark mode
• Ensure everything looks consistent

Release

• Updated the CSS to complete the move to using CSS variables for colors

[SylvesterWilmott]

Thanks @sandymcfadden, the differences look good to me!

[codebykat]

Reviewed code and smoke-tested, this looks good to me 👍