Hi ​@safiya - It’s only possible to have one Theme configured in the Email Campaigns module at a time.  The idea is that the Theme is a basic style that is universally applicable for all your email campaigns.  It might, as it is for us, simply be used to define a table for content with a certain width (and a background colour surrounding it) and a company logo at the top.  Working within that Theme there are still many ways that an individual campaign could be styled. If you’re considering some very different styles, you always have the choice of working within your universal Theme (which is what we and most folks will do) or you can also choose to adjust the Theme if needed.

thanks! ​@Kenneth R this helps.