It's not too difficult. It is just a question of altering the colours and table widths of a current theme, and creating a new logo if you want. And also creating new icons if you want to.
The easiest way would probably be to load the index page or forum topic page into your web page creater and adjust that.
Here is the theme details of the TiBB Theme. Click on thumbnail to view...
You can match those colours to the ones you see on the page.