to get to the themes, right click an empty part of the desktop and click properties. here you can change the themes, screen savers, desktop screen size etc.
regarding the language settings, open control panel, double click "regional and language options" change each dropdown box to English (United Kingdom) , apply your changes and that should be all you need to do. (personally i add uk keyboard and delete the US keyboard, accept the warning and reboot your pc, everything then should default to the uk settings.
double click the mouse settings in control panel, check all the settings in there and see if something looks wrong (cant help much as i have a microsoft optical mouse) and the settings may be different.