This is how you can utilize some of the defaults we added into Material for customizing the node for adding things like images and colors. You always utilize the node grid as well which is covered on the UI.X documentation.
First lets navigate to Application > View Node Tree > scroll to the node > Node Options > Style Options
Next, you'll want to add the following class that will append To .nodeInfo to turn it into metro styling: isMetro. This will be need for all nodes requiring that they have an image or a different color.
For color, add one of these classes:
For an image add the following class next to the one where you set isMetro: metro-image
Next, you can go and tab over to Styling. To add a background image add the url to the background image CSS property.