Full instructions on when and how to use the Base Skins can be found on the download page, please ensure you read the page in detail before downloading the Base Skins as it will save you problems in the future.
Currently there are four versions of the Base Skin - two light and two dark one of each solely based upon the Default FM2012 Skins and another one with the Background Changes re-enabled thanks to Radestock.
If you have any further questions on the Base Skins please ask any questions in the Skinning Hideout on the Official SIGames Forums.