The "don't DIY" option hasn't been explored here: why not pay a graphic designer?
You might still choose to bring in a design specialist for pixel-perfect work or usability perfection, but it's no different than having the hacking chops to get your prototype into a functionally complete form. Done is better than perfect.
“I’ve been amazed at how often those outside the discipline of design assume that what designers do is decoration—likely because so much bad design simply is decoration. Good design isn’t. Good design is problem solving.” – Jeffrey Veen
It certainly wouldn't be worth hiring a designer just to get "pretty" pixel perfection. I would hope they could provide thinking as well. By doing the work in parallel with you, they should make it possible to go faster than doing it yourself.