Missing Support for Flexbox, Grid, and other CSS properties

Wondering how I might help on getting this up to date?
You are most welcome to take a stab at this in a PR. I haven't used this lib in a while as I don't use stylus anymore, so I myself am a little out of sorts with how everything is setup. If I recall correctly, I believe we rely on the built in css.vim for a baseline but had brought in a number of styles that weren't supported (at the time) in the syntax file itself. They are likely 6-years out of date :D which makes sense given the properties missing you mention. It may just be finding another project with updated css syntax and copying them over to this project's syntax groups.
Hey @wavded if you were willing to show me around the code a bit, so I understood it, I'd totally be willing to update the code so it supported modern CSS features. I use this vim plugin all the time, so I'd be more than happy to make the updates for myself and then set up pull requests.
I need a little help though. I'm not familiar enough with vim syntax highlighting and such.
@wamoyo Thanks for your willingness to help out! I don't have much time at present to look at this. I am rusty at best on how to jump in myself, so I don't know how much I'd be able to help without relearning things myself. I based this on the vim-jade (now vim-pug) plugin when I initially worked on it:
https://github.com/digitaltoad/vim-pug
There may be things to glean there on how they updated things (although HTML elements don't change as often as CSS).
@wavded Hmm... cool. Is there a resource you used originally to learn how to edit colorschemes for neovim? I would just need some support getting started. I might just fork in that case if that's okay : )