It's allowed as far as I know, but you need to make sure that your customers respects the acceptable use policy: https://www.99stack.com/legal/acceptable-use-policy, either by linking to it directly or just copy the text.
Regarding branding and stuff like that, I think it's the same as for the affiliate program. I.e you're pretty much obliged to use your own brand. You might want to contact support if you seriously considering becoming a reseller.
