As I understand the question - it's not about font color nor font background - but how to change editor background which is for me (version 2015 sp1) blueish gradient.
I have gone thru the "options" and "customize" menu and there is nothing in that regard.
Their sister product UModel can do that by finding "Styles" docking window and looking for "Diag. Background Color" and setting it to e.g. "White" instead "gradient".
Unfortunately I don't see equivalent feature in XML Spy.
Also, I quickly looked at the exe file where I saw plenty of RGB values stored, I don't know does that mean the value is hard coded or it's just complicated to find background image settings.