Unfortunately, I have been unable to reproduce the problem that you describe so I'm not sure what might be causing your problem.
I installed a fresh Showkase v1.7.3 site.
I changed the theme to Kosel.
I changed the 'Max site width (px)' to '1000'.
I changed the 'Overall color scheme' to 'custom'.
I changed the 'Background color' to '808080'.
I created a 'Basic' page.
The entire background of the page I created (the body content and the surrounding area) is #808080.
Here are a few things to check and try.
(1) If you are not already using the latest version of Showkase (v1.7.3), then please try upgrading your site to ensure that any bugs from previous versions which have since been fixed are not contributing to your problem.
Full instructions for downloading the latest version and upgrading an existing site can be found on the Upgrading Showkase support page.
(2) Try clearing your browser's cache before reloading your site to make sure that your browser is using the most recent versions of all your site's files from your web server (and not any cached versions).
(3) Check to see if you have any custom CSS (perhaps in your theme's 'custom.css' file) which might be conflicting with Showkase's own CSS and overriding the background color set in the interface.
I hope that these suggestions help.
If not, then please post back with the URL to one of your web pages which demonstrates this problem so that I can see the problem for myself and hopefully help further.