A family of Microsoft word processing software products for creating web, email, and print documents.
The screen shot in your first post shows that you're using Word Online in your browser. The theme you need, and the Design ribbon shown in Doug's post, are only in the desktop version of Word.
In Word Online, click the Editing button to the right of the Help menu item, and then click Open in Desktop App (this will work only if you have the desktop version installed). When the document opens in the desktop, you can apply the Ion theme. If you then want to go back to working in Word Online (although I'm not sure why you may want to), save the document on OneDrive, and the theme's formatting will be there.