You're probably not gonna like this, as it would mean editing all your pages (unless you've used templates).
Enclose the whole page content into a fixed-width table, and place your background image into this table (as a background image - I've borrowed & trimmed yours to fit a 760 pixel table). Then create a single colour image (matching the dark blue/black at the outer edge of your current background image) and use this as the background image for the page itself. Like this.
You could change the body text/font to a lighter colour (white or a yellow tone). You might also want to change the colours used for your links for ease of use. (CSS is a great way to control rollover colours and styles.)
Text/font colour is always an issue with graduated backgrounds, but good luck!