Yes, Lucius is a css superset that supports nesting as per LESS [1] or Sass (SCSS syntax) [2], and also has normal hamlet-style variable interpolation. For those interested, there is a comparison of these 2 css superset languages that goes over main features [3]. Much of the benefit of SASS at least is derived from re-using css libraries (in an easy way that is not possible with normal css) [4].
I think Lucius should somehow seek compatibility with sass to allow re-use of css libraries if this is possible. One approach to this is to simply use Sass instead of Lucius- and we could easily put hamlet style variable insertion on top of it. Lucius will always be needed though to have only haskell dependencies for an easy initial install and usage of Yesod.