Not fully. You can't customize Fabletics outfits or bundles, but you can swap an item for a different color of the same style. During select Build Your Own (BYO) promotions, you can mix and match eligible styles, such as choosing your own top and bottom for one VIP Credit.
BYO offers vary by promotion. Some let you pick any two eligible styles for one VIP Credit. Check Fabletics.com for current BYO promotions.