Allow basic theme changes to be done from Django Admin

[untari]

Work in progress for issue #343. Do not merge yet.

This PR is here to show my progress and be able to receive feedback on my work.

Task list

• Extract colors, fonts and Font sizes being used and convert them to CSS variables. These values are in CSS files as well as embedded in JS.
• Allow modification of these variables from the backend from ZubHub settings. Add a new item called "Theme" in the area below (see issue Allow basic theme changes to be done from Django Admin #343).

Breakdown into steps

First try for one value

• Get familiar with Makefile commands.
• Get familiar with codebase.
• Identify one color variable you can modify.
• Create a Button for it in then admin panel.
• Add code, so the Button modifies the color value'.

Extend for all the CSS values

• Extract colors, fonts and Font sizes being used and convert them to CSS variables.
• Add them to the "Theme" Item.

[Ritsyy]

Let's choose a new URL for this

[untari]

Hi @Ritsyy I was thinking to name it api/theme. What do you think ?

[Ritsyy]

we can replace this block with -
self.get_queryset().last()

[untari]

Hi @Ritsyy I just replaced it. Thanks