You would need to modify the templates to tweak your style to exactly how you want it. Most notably these would include
- postbit
- forumhome
- navbar
- header
- footer
To create the gradient for the category strips you would need to make this in photshop or a similar package and upload to your server making note of the location then in the category strips section in the CSS add to your existing hex colour code ' url(path to gradient)' and that will then put the image in there.