@vinceliuice user produces some really themes.
I have used the users theme.txt as groundwork for several Manjaro themes. I have even provided updated Manjaro png’s for the project - so I know it works.
The various arguments for the script is used to select the background and to set the positions of various elements - nothing fancy really - but putting it together is hard work.
Your problem seems to be hardware related. You can’t force it to an unsupported mode - so to get the best result you should try auto - this will pick the highest available resolution.
GRUB_GFXMODE=auto
If it don’t you can try with the resolution provided by videoinfo
GRUB_GFXMODE=1280x1024