Unfortunately, the grey background of the page cannot be changed. It's caused by incompatibility between the forum code and the Amiga browsers (IBrowse&AWeb).
What you can do is to change the look of the browser interface itself like buttons, fonts, background graphics, etc. In IBrowse press the right mouse button and select Preferences -> MUI. Similarly you can change the look of other applications that use MUI for its graphical interface. Or you can change the default look of all of them by changing the main MUI program itself (StartMenu -> Preferences -> MUI).
However the MUI program is shareware which means its configuration abilities are limited until you register it at http://www.sasg.com/cgi-sasg/order_info?app=mui
I think it's worth to register the program, eg. compare the default look of IBrowse and SimpleMail in your AmiKit installation and this screenshot
showing the customized look of IBrowse and SimpleMail buttons and graphics (enabled by registered MUI).