ProStyles now includes three new child stacks:
Backgrounds, Overlays and Visibility that can be used with other BWD stacks such as SectionsBOX and upcoming releases.
If you are using SectionsBOX or other BWD stacks that allow backgrounds and overlays etc to be applied via the Stacks 3 add child button (the blue + icon) then you must ensure that ProStyles is installed in your library.
You do not need to add the ProStyles stack to your page, just ensure that you have it installed, this is because the ProStyles stack package contains all the other stacks inside it.
By installing ProStyles you are also installing the Child Stacks that the other BWD stacks can access.
Of course if you want to use Master Sytles you need to add the ProStyles stack to your page and add a Master Style.