Add /css-var slash command

[Wolfsblvt]

I made it on purpose that you can only set actual variables.
I don't think overwriting CSS properties via slash commands is good.

But this way, you can just use Custom CSS and then dynamically change the vars for those that you created.
Or overwrite a few of the default ones, like the examples that I have provided.

Changes

• Add /css-var slash command to set CSS variables on some elements
• Update gallery div id to something actually unique ("#draggable_gallery")

Checklist:

• I have read the Contributing guidelines.

[Cohee1207]

Nice to have in the future: an enumeration of common CSS variables.

For reference: https://stackoverflow.com/questions/48760274/get-all-css-root-variables-in-array-using-javascript-and-change-the-values

[Wolfsblvt]

Nice to have in the future: an enumeration of common CSS variables.

That's actually a pretty cool idea. I'll note it down for when I have time.
Maybe I can even check if you could get the var value of a CSS var for an element. Shouldn't be too hard. Worst case it's manually traversing the DOM up until you find it. Then we can add a slash command for getting the current value for those as well.