Unable to change the Theme of form?

Secondly, we can't change the themes of the forms too. Exemple, i choosed a theme, then i clicked use theme and choose one of the form, but then nothing happened...then i tried "install theme", it turned to a page like "installing" but then it stayed at this page very long time and blocked. 

Have you tried clearing your form's cache and your browser's cache?

If that doesn't work, here are the steps on how to change your theme, I went ahead and cloned one of your forms for testing and demo purposes.

1. Click the "Preferences", change "Themes" to "Default" and delete the code in the "Inject Custom CSS", assuming you don't have your own CSS code there and you want to totally change your design.

2. Click the "Themes" option.

 

3. Here you can select the theme in your "My Themes" or in the "Theme Store".

 

If that doesn't work, have you tried using a different web browser or updating your browser to the latest?

Do let us know if that works.