Well at the moment I can find many many PHPBB themes however as for MyBB I can't find any that are great, although I love MyBB it actually creates a much better community so are there any themes for MyBB that look professional as I have not found any in the "Mods" section.
There's plenty of professional looking themes in the mods section. If none of them suit, make a request.
Not to tut mine and other mybb designers horn but there are many sites such as...
Jessie S:
http://dynaxel.com (Free Themes and Paid Premium Themes)
Infranight:
http://zynge.net (Paid Premium Themes)
Combus:
http://indiepaper.com (Free Themes)
(too lazy to type the rest, looks at the stuck thread for more.)
Which provide some of the better themes for mybb, however there are still a few nice ones in the Database.
If you have a phpBB theme that you like, someone can code it into MyBB for you.
ATM I'm trying to create a couple free mybb themes that are as sexy looking and web 2.0 as current vB themes being released, bare with me.
I have to +1 to jessie he is a great theme maker.