custom.cfgcolor_normal=light-gray/black
nano /boot/grub/custom.cfg
color_normal=light-gray/blackmenu_color_normal=light-gray/blackmenu_color_highlight=light-cyan/black
Save it = STRG+O & STRG+X
detailed source:
https://help.ubuntu.com/community/MaintenanceFreeCustomGrub2Screen