I haven't put together any documentation on it yet unfortunately. The most reliable place for examples is actually the runtests.hs program in the Hamlet repo. But a very quick intro: you can use #{var} and @{url} interpolation (I think it should work anywhere). Syntax is standard CSS, with the single addition for the moment of nested blocks. That means that:
foo, bar {
baz, bin {
color: red;
}
}
Results in:
foo baz, foo bin, bar baz, bar bin {
color: red;
}
I'll hopefully update the docs after 0.8 goes out.