use dynamic themes in Framework builder.
when a user logs in, their ID is stored in a session variable, you can use the ID session as the variable to load the theme.
Create a theme for the user, then apply it to the page using dynamic themes. Go to WebAssist -> Framwework builder -> Themes -> Apply Dynamic Theme. For the trigger variable, select the ID session variable. Click the plus button to add a theme, select the theme to apply then enter the users ID value that it should apply for.