Like what the other members said and a little more. Firstly, elaborate on your theme by giving the default stock some appropriate images/icons so guests, members and anyone else know and get a feel for what your forum is all about. I noticed many images have not been changed that are incorrectly not saved for the background they are being applied upon like the collapse icons and the sortasc/sortdesc icons. There maybe other images with redundant backgrounds, I haven't checked so ensure that you have checked for incorrectly saved matted images on your forum.
Like the others have said, colour usage is poor, and like ChrisLM2001 said there are free colour scheme tools to help you in your endeavor to implementing better colour implementation which I have listed below.
Goodluck with your site.
1. http://www.defencemechanism.com/color/
2. http://www.steeldolphin.com/color_scheme.html
3. http://www.colorblender.com/
4. http://wellstyled.com/tools/colorscheme2/index-en.html
5. http://slayeroffice.com/tools/color_palette/
6. http://meyerweb.com/eric/tools/color-blend/
7. http://www.mbcomms.net.au/tools/