Hi darksabersan,
I don't know if quotes around resolution may give problems... Maybe try to remove them and update grub.
Have you tried modifying the grub.cfg for the desired resolution? (Yes, it says not to edit it, but we like low level tweaking ) Just for testing (if you forget to edit it back it will be always rebuilt with configuration files parameters next time, so don't worry, or just update grub).
On my laptop I built a grub.cfg from scratch, in which there are: set gfxmode="1366x768"
set gfxpayload=keepMaybe you can try to edit yours putting your values for these parameters.

You can try pastebining your grub.cfg, anyways it can also be an hardware issue needing some tweakings into grub files to support that resolution, but i can't say that... Try to search online how to test for supported resolutions and modes.

Have fun with BackBox!

after making any changes to grub, you need to update grub for them to take effect

Code: [Select]

update-grub

Exactly 
darksabersan, i assumed you updated after modifying... don't tell us you didn't do that 
Anyways with grub customizer there should be no needing to update grub via terminal, because, as i can see, it automatically generates a new grub.cfg when you click on save button. So in this case the issue seems to be present (unless you didn't save  LOL).
But i guess you can solve searching around the web.

Have fun!