Module Variations or Module Hilites are a facility to individually style 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 then adjusts the styling to the prebuilt suffixes.
Enter any available suffixes at Extensions → Module Manager → Module → Module Class Suffix
The following positions: main, main2-main9, user1-9, left, right, inset, inset2 & main-feature; have support for the following module class suffixes: hilite1, hilite2, hilite3 & ad.