No, you cannot change the theme of a new survey created by you in a shared folder if the folder owner is different. To make changes, you need to request "Can Copy" access from the survey owner to duplicate the survey into your account. Alternatively, you can create a folder in your account, share it with the owner, and ask them to move the survey into your shared folder.

Helpfile of Share folder: https://www.questionpro.com/help/share-folder.html

Helpfile of Copy Survey: https://www.questionpro.com/help/copy-survey.html