Background patterns

To add patterns you'll have to use the  gradients field, because that one pastes pure css code inside the background's style. You would have to add something like this inside the dedicated area for Custom CSS Code under the Theme Options Panel

background: url( repeat 0 0 transparent !important;           -webkit-background-size: initial !important;

This code would add an image which repeats itself as the background for any project / gallery.