Motherboards have heatsinks on it and you wouldn't want to cover them. You need to dissipate the heat. You can buy aftermarket heatsinks and stick them on with stuff like Arctic Silver Thermal Epoxy. But nowadays motherboards are so well built aftermarket chip heatsinks are not required. I'm afraid you are out of luck. Any money spent on a cover to change the look of a motherboard would be better spent on changing the motherboard itself.
Perhaps making custom colored sleeved cables with some Paracord 550 would be of interest to you.