It is made from a primitive boolean control (flat square button, I think).
- Select Customize the control
- Change to Customize Mode
- Right click and select Picture Item you want to change
- Then use Import from File to go get your icon
I recommend using PNG graphics, which support transparency. This On/Off toggle actually has transparent pixels in corners so it has a rounded look no matter what is in the background.
Here are the original graphics I used for this control: