Hello
The header of our Arkadia template use a single image which you can edit directly. You will find it located at
http://joomla.noobinator.de/templates/j51_arkadia/images/bg_bar.png
To edit the content background color try adding the following to the Custom CSS field of your templates parameters (edit to taste)...
.wrapper960 {
background-color: rgba(230, 240, 240, 0.8);
background-image: none;
}
CiarĂ¡n