Hi JamesJoey,
If the themes that you have has an extension of .themepack, like the ones that you can download from the Microsoft website, once you open the themes it opens under the “My Themes” section. You can then save the same theme for your later use.
Refer these links for more information:
Wish you were here: Sharing themes with friends and family
Regards,
Divya R – Microsoft Support.
Visit our Microsoft Answers Feedback Forum and let us know what you think.