1 - Go to the "Documents" folder
2 - Open the "SimBin" folder and then the "RACE 07" folder
3 - Open "Config.ini", press CTRL+F and type "MENUSKIN="
4 - After the equal symbol, choose any of these values:

GTREVO for GTR Evolution
STCC for STCC: The Game
RACEON for RACE On
MOSQUITO for Race Injection

5 - Save and exit

To revert to the default RACE 07 skin, just delete the value that you inserted into "MENUSKIN=".