Customizing legends is a piece of cake with Map Mavin. In the video above we break it down step by step (not that there are many steps!).
When in the Admin panel, click the Layers tab.
Select a layer (the example used in the video is "SUBWAY STATIONS")
Click the layer and you'll be brought to the layer editing screen.
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.