If you haven't changed the default desktop theme by editing (and the saved it as default), you could select it again..
Or you should be able to reinstall the desktop theme or the desktop to reset the standard (default settings), or you could check your profile under /home (a hidden file, dot ( . )) and check its contents to see what is being loaded: which would give you the likely names for items, desktop, theme etc..
mate or cinnamon is the desktop system name(s)http://www.techsupportalert.com/content ... mize-Theme
You may want to save your whole /home directory(folder) including hidden (. dot files, can be shown with the file manager) in order not to lose any item(s) you may want to retrieve again..