Once you have a fully designed email with a design theme applied, its easy to save and re-use these.
1. Go to My Content, find the email design you want to re-use, hover over it and click the duplicate button
2. Name the email template
3. To organize it, create a folder in My Content called Templates, and move your new email template into that folder
Now, any time you want to re-use that template, go to your Templates folder and click Duplicate on the email you want to re-create.