My advice is NOT to mix operating systems on one harddisk. Just to avoid frustrations for you and for me. I have a desktop and can easily switch my ssd’s to have ArcoLinuxD Cinnamon to ArcoLinuxD Awesome etcetera.
But sometimes you have no choice then to put two operating systems on one harddrive.
Let me give you a tip.
Grubcustomizer is standard installed on your ArcoLinux.
Grubcustomizer is not really there to change colors or give you a nice wallpaper when booting shown here.
Grubcustomizer is there to easily change the boot order when dual booting.
To set Windows 10 back as the default OS to boot into.