If all you want to do is change the
colors, that's easy... look in the Player Window code for the Exp and AA Exp Gauges and change the color settings for each; colors are specified by values of Red, Green, and Blue between 0 and 255, the larger the number, the more of that color you get. Can't remember the color code for the default XP bars, but if you look in your Default Player Window code, you should be able to find it; just make the two match.
If you want to change the actual graphic, that's a little more involved. You'll have to include a tga in your custom folder that has the XP bar graphic (or add the XP bar to an existing tga), then edit your Player Window xml to make sure that tga file is specified as a usable texture, the location in the file of the gauge, and call that particular gauge to display instead of the existing one.
It's not nearly as hard as it sounds
but there ARE a few steps to it. You might PM the author and see if he'll do the graphics and recoding for you.