Support for css variables in RCSS

[Illation]

Hi, what would it take to support CSS Variables?

They where mentioned in #22 a few years ago, but I don't believe there ever was any follow up.

I'm working on writing a tool framework for my engine on top of RmlUi, so there will be lots of different UI elements that should ideally have a consistent appearance, and I imagine all the different colours will get confusing and hard to keep track of very quickly.

At a minimum just the ability to make variables for colours in a global context would be sufficient for me, although long term it would be fantastic if they supported lengths and played well with media queries or could otherwise be controlled from c++ as well to support theming.

I'm happy to look into this myself if I'm pointed in the right direction and the scope of change isn't too huge