You cannot use CSS GUI styling with LuaRT
ui module, out of the box.
You will need to code you own CSS layer that will interpret CSS styling to widget properties/methods.
I didn't know about ZSS and after looking at the code on GitHub, this module can do the job to interpret CSS to Lua table but will need some additional work to provide the
ui module interface.
If someone is interested, here is the link to the ZSS project : [Login to see the link]