Sorry, I didn't actually explain it very well, so I'll start again
For a start, it's line 335, not 355 in template.css.
Now, the options you have are this -
1. Edit the s5_header_bg.jpg image currently in templates/eshopper/images. It's currently black, so editing it in Photoshop or whatever, and overwriting the original will change the color on the site.
2. Alternatively, make the section mentioned now read this -
Again, you'll need to make the #FF0000 whatever you need it to be. I just use bright red because it's obvious when it's been changed.
The second option is a little more brutal, as it will fill the entire width of the screen, not just the main body column with white borders.
I hope that makes more sense!