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 11 suffixes: tab1-3, bg1-3, underline, number and flush, flushtop, flushbottom.
The Tab suffixes add a solid background image behind the header, in the shape of a tab. The BG suffixes change the color of the module background only. Underline simply adds a border to the bottom of the module header. Remember, you can always combine suffixes together such as tab3 b2, which will produce a colored module title, with a colored background. Number is specifically designed for the menu module, adding a decimal list before the border graphic.
Flush will disable all padding and margin to the module for a tighter fit, with FlushTop and FlushBottom disabling the margin/padding for the Top/Bottom parts of the module respectively.