By default, shapes don't have a fill color when you add them to your whiteboard. Choose any color to fill out your shape. When you want a shape without a fill color click on the upper left on the square with the diagonal line.
Border color & border size
Adjust the border color and the border size of the shapes based on your preferences. When you don't want to display the border click on the upper left on the square with the diagonal line.
Add text to shapes
To add text to shapes make a double click on this shape. When you are done with typing click somewhere out of the shape, and you can adjust the text with all the standard text options.