It's a workaround that's not officially supported and I should warn you that on some systems the light-grey setup doesn't seem to work optimally (in some cases not all buttons show but this issue might be solved by minimizing and then maximizing the program window so that the UI gets redrawn).

That said, it's a very simple operation: go to C:\Program Files\Xara\Xara Designer Pro X\12 (depending on what version of Xara software you have), locate xaradark_theme.xml and move the file to another folder. Make sure you keep it in a safe place because you might want to use it again. When you go to Window/Control bars... there is the option to select large or small buttons.