The custom shape tool contains a library of shapes for building up characters and scenes. These include a handful of basic shapes as well as sets from Pitch Dev Studios and Silver Drawing Academy.
In the settings panel to the right, you can set it to fill to create a solid shape filled with color.
Setting the type to stroke instead will create an outline of the shape with your selected color.
You can increase the thickness of this outline by increasing the stroke width value.
Decreasing the opacity will make your shape or outline more transparent.
Holding Shift will fix the scaling size ratio of your shape. You can also check the box for this in your settings panel. Holding down Alt while dragging your shape will center it around the starting point where you clicked.