Select a style from the list of options in the
"Styles" menu.
This menu does also allow to save style settings made on your PC.
Import a style using the "File - Import style"
dialog to add new styles. These will be automatically added to your list of
styles (see above).
Develop your own settings in "File - Tree
properties..." in the "Styles" tab. You have a vast number of options to
influence the look of your decision tree and may use the inheritance system
along the option hierarchy.
There is a way for you to make it easier to choose the correct tree element when you assign style settings. Simply right mouse-click an element of that type and Insight Tree will lead you to the right node type in the "Properties" dialog.