Moving from a design to implementation on a certain theme

Hello,
I’m trying to understand something, so I’d be happy for the help of this forum.

I really like the suffusion theme, because of the flexibility it gives. But till as much as I tried designing the with this theme, it still looked not so great.

I want to turn to a designer or preferably a developer who can desgin a good looking website for me. I understand that from a sole graphic design I can move and ask someone to built a new theme for me. what happens if I insists that the platform would be suffusion? How would they implement a whole new design (that suffusion might not have all the functionality built in for it)?

If I choose a certain theme, am I limited to what it offers? as far as I know, I’m not, I just try to understand how to move from a pure desing to implementation on a theme.

Thank you
Shlomo