Modern ShortPoint Theme Builder is only for Modern SharePoint pages so when you open Modern Theme Builder on classic page, you will get two options:




Option 1: Switch to Classic theme builder

 

Clicking on Switch to Classic button, ask you to discard any changes (if there are any) you did in Modern Theme Builder and re-activate Classic ShortPoint Theme Builder.


If you don't want discard your changes, you can publish your Modern ShortPoint Theme Builder changes and open Classic Theme Builder from ShortPoint Dashboard.


Option 2: Navigate to modern page


On the preview pane right side of your page, redirect to Classic Page.