Customizing legends is a piece of cake with Map Mavin. In the video below we break it down step by step (not that there are many steps!).
When in the Admin panel, click the Layers tab.
Click the layer and you'll be brought to the Layer Editor. From here click on the Styles tab.
Scroll down to the Legend tab.
From here you can decide both the fill and surrounding stroke color for your points and polygons and stroke for line files.