I saw a post titled 'Changing color for sections' that lacked a clear answer, nor is it addressed in the documentation. [ edit: Here's the solution: ]
add a module class suffix, eg " section3" (note the leading space) and the row will acquire the parameters you specify in ThemeMagic.
note, you can stack suffixes: " hero-unit section1 myOwnClass" (again, note leading spaces) so you can do specific things in your css.