Css color consistency

[Arnei]

Includes #1400. #1400 Should be reviewed and merged first.

This PR replaces the "hardcoded" color values that are flying around everywhere in our scss files with the color variables defined in _variables.scss. Color values that are not represented by our variables are either added to the variables file, or replaced with a variable with a similar value.

Goals:

• Make colors easier to work with. Having our colors all grouped in one place should make it easier to e.g. implement a dark mode.
• Reduce the number of different colors. We really should not need 50 different shades of grey (number approximated).
• Increase contrast. I used the opportunity to slightly change some colors to make the ui more readable.

How to test this

This touches every element that has color, so unfortunately pretty much everything. Quickly click through everything ui element you can find and check if any colors appear "off".

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-1431

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-1431

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/1431/2025-12-01_13-20-35/ .
It might take a few minutes for it to become available.

[ferishili]

Thanks @Arnei,
Your changes look fine, however a few things need your attention!

First and most important one is:
While running the UI click-through tests, I noticed the contrast in the calendar (month and year selection) looks off, as shown below. Could you please take a look at that as well?

[ferishili]

I noticed there's a whole block of code commented out. Is that intentional?
If so, could you please add a short comment above it explaining why it's being kept? Also, it would be better to use a proper block comment instead of multiple single-line comments for readability.