Its been really frustrating that I haven't been able to change Grub's background and have finally found the problem which is gfxmenu=/etc/grub/message.mint . I put a # in front of this line (so grub ignores it) and the GRUB screen was black with a blue box containing OS options - I guess this is normal Grub.
Now, how do I configure the message.mint file? I want to get rid of the background and configure the text colours to contrast with my made background. If only Linux Mint have a GUI to customize the gfx settings.