How easy is it? Customizing a message board can be as easy as choosing a theme and merely adding a banner. In difficulty degrees, the answer is harder to gage, but I would say the degrees are, roughly speaking;
1. Single panel designs
2. Left Sidebar designs
3. Right Sidebar designs
4. Surround designs
Skins are design layouts where all the coding work is written in advance, such that a user can copy and paste codes in the relevant areas and have a working message board of any difficulty level without having any knowledge of coding.