Migrate users.css to users.styl

**users.styl** should compile to CSS that is nearly identical to **users.css**.

The only major difference is selector specificity. In **users.css**, a selector like `#main-area` might be used in one place, and a selector like `section#main-area` might be used somewhere else. Because Stylus groups similar rules together, one form is used consistently throughout the compiled CSS.

I suspect some improvements are needed here -- don't be afraid to be honest. I am especially confused about how this will interact with the existing **users.css** (maybe I should be using more `compat-only()` and `compat-important()` calls?) but with some guidance I should be able to correct issues like those before long.