From your page source code, I see that you are using the CLASSIC template.

You should create your own template system before editing code, so think about this...

Anyway, what I propose is not a code edit...

In: templates/classic/images

you will see a file called header_bg.jpg

FTP this file (image) to your hard drive (in binary mode, as it's an image)

Open the image in an image editor (even MS "Paint" will do).

Using the edit tools in your image editor, make the entire image white.

Save the image (as header_bg.jpg)

FTP this new image to: templates/classic/images

When prompted by your FTP, OVER-WRITE the existing image.

You now have a plain white header_bg as a background.