I don't see anything unusual. My guess would be that until recently, you have had your screen set to 800x600, and then you changed it. So now you see more of the background color. At 800x600 it is just visible above the banner. Don't worry about it. Anyone using more than 800x600 while surfing will expect something like that, and the color goes nicely with the rest of your page. You can just reduce your window size to lose the "stripe".

(Or possibly you are talking about something else that is not visible in my browser, if the above is wrong, tell us more)

Thanks Corwings.
I have not changed my window size recently, but my computer was taken away for adjustments so perhaps the technician did it.
Cheers
Tony

As larger screens become available it is necessary to make sure that pages scale gracefully. The old idea of simply letting them expand or contract ad infinitum falls down as line lengths reach a point where they are too long to read comfortably.

One design that I have experimented with is a fixed width, centred, page on a plain background with a background image filling the empty space on either side (see link).