There are a number of advanced settings for the cartoon representation that you can control to make your image more informative.
Width and Thickness
These two buttons control just what they say, the width and thickness of the cartoon view. These features do not work on all the cartoon types so if you keep trying to change the thickness and it is not working, tyr changing the type of cartoon being displayed. (See below)
Transparency
Transparency is the amount that you can see through the cartoon image. The higher the transparency value, the more ghost-like the image appears, while the lower the value the more solid the image appears.
Tube Radius
This feature works only when the cartoon type is set to 'Tube'. Increasing the value will increase the radius of the tube while decreasing the value will decrease the radius of the tube.
Type
There are several different cartoon representations that you can use when working with your molecule. The type 'Skip' simply hides cartoon for the area which you have selected.