Using dynamic Themes is the same as using Static Themes up to the point when you apply it to the page.
When you got the point where you would select the WebAssist -> framework Builder -> Themes -> Apply Theme, select he Apply Dynamic theme option instead.
In the Dynamic Theme Window, select the user ID session value for the trigger variable.
Then click the plus button, And select the them that will be used, then type the value that will trigger that theme. This is the value for the userID session variable that should cause that theme to be used.
Repeat for each theme / value combination.