Since I discovered less css my life has changed!
Variables, includes, overrides, structured css is magic!
I think also jquery ui theme development would be simplified a lot:
let's say you have a base .less file for themes: each theme could change only some of the variables, leaving the default values for others. A great improvement imho.
Furthermore, developers could integrate their customizations with another custom .less file, and the final .css would be very clean, without having to modify source jquery ui files or having to use !important...