Yes, you should change the css.
If you need the blue over the full width of the template then you could add the following code to the template.css (add these to the end of the file):
#ja-header {
background: none repeat scroll 0 0 #0000FF;
}
If you just want the blue over the width of the content then you should add the following code to the template.css:
#ja-header .main {
background: none repeat scroll 0 0 #0000FF;
}
In both cases the color is pure blue. Change the color to your liking.
Regards, John
PS. If you want to do more customization you better create a separate theme (new directory in themes directory) with in that directory a css-directory for your css-customizations, a html-directory in case you need to change the behaviour of the php-code and an image-directory for your custom images). In the root of that new directory should be an info.xml file with the name of your new theme.