Giving your forum an entirely new look and feel is easy — just download one of these great themes! There are even themes for the admin control panel.
Projects in Multiple Color Choices

Fully responsive, modern & multicolor theme for MyBB 1.8 (Documentation included)

Fully responsive, modern & multicolor theme for MyBB 1.8 (Documentation included)

A basic Responsive MyBB theme using Bootstrap 3

Focus is a clean and Elegant Flat MyBB theme which can be used by all sort of community users, one of the cool features of this theme is shrinking header on scroll. Focus MyBB Theme has been tested thoroughly on all browsers.

The Agood theme was designed for MyBB 1.8.21+ series.

Simple and clean theme with 5 different colours

Free Responsive MyBB Theme - NetPen
Which also already for IPB / Xenforo exist .. now however also for Mybb