Changes made on the Designer doesn't show during Preview.

I can't get the theme to apply to my form. In the Designer mode it says it's all good but when I preview the form it won't show up.

I would like to apologize for the inconvenience this have caused you. I would advice you try to clear your browser's cache and check it again. 

On the JotForm Theme Designer:

Then, 

Preview mode: 

You can also check if changes made on the Designer was properly saved by checking the "Inject Custom CSS" of the Preferences toolbar. It should have codes there.

I hope this helps. Let us know if you need any further assistance. Thank you.