Got it! Google AEButton Plus to remap the volume keys.
Then google XIAMultitheme and download that, install it and run the configuration file for it, on the start menu tab click refresh start menu links. This will create shortcuts to increase/decrease the backlight.
So just use AEButton Plus to assign the volume buttons to the XIAMultitheme shortcuts and that's it.
There's probably a much more efficient way to do this (without any programs)...but I don't know it
