I would like to try applying a theme but if it doesn't work undo it.

Preview isn't enough. I am nervous that if it doesn't work I may not be able to remove it, or will have to resize a lot of the form, redo tabs etc

If you have applied a theme on your form and you don't like that. Then you can remove it from the "Themes" page, and nothing will be loss other than theme.

I would also like to update, you can get your old form, from the Form revision history, please check the below article for doing that:

https://www.jotform.com/help/294-How-to-view-form-revision-history

If you face any issue, or need further assistance, feel free to revert back. Thanks