Every time you create a new widget, there is a default icon assigned in the upper right corner of your device's variables. This icon is a simple visual reminder to the type of sensor you are working with (energy, thermometer, graph, directional, etc.). 

Change Default Icon

Go to Devices and open the device whose icon you want to adjust. Each variable has a default icon displayed.
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To change, click on the icon and select a new icon to display.

Choose one from the options: 

The final result will be similar to the one below.