The visual settings for the ULX Menu is in the Client Settings in the GUI in-game on your server. No .lua file editing needed.
I think we want more than that? To change the colors of the menus, without editing the code, you would have to make a custom skin and set it in that menu. I believe Stickly was even thinking about phasing out skins (not sure on that).
Otherwise to manually edit everything in XGUI, that could be a bit of pain. But if you really want to, you can find everything in
ulx/lua/ulx/xgui and some stuff in
ulx/lua/ulx/modules/cl.
EDIT: I would recommend starting in
ulx/lua/ulx/modules/cl/xlib.lua.