Module Variations allow for the individual styling of each module within a position, without it being global. Code wise, this is achieved by adding a class into the module code which loads from the Module Class Suffix field in the Module Manager. The theme CSS adjusts accordingly.
Enter any available suffixes at Extensions → Module Manager → Module → Module Class Suffix
There are 3 standard suffixes: titlecolor, simple & background. All positions can apply the titlecolor suffix, and simple & background only apply to the maintop, content-top, content-bottom and sidebar positions.
However, there is also a non-standard suffix showcase, which applies to the showcase-a/f positions. This affects the module title as seen on the frontpage, i.e. Popular