By default, all emails share the same theme. Themes consist of the header, footer, and all theme settings found within the editor.
That means you only have to make a change to one of those areas once, like which logo to use, and it will automatically be updated in all other automations and draft/scheduled newsletters. Any sent emails are not affected. This shared theme can be a big timesaver as you want your emails to have consistent branding.
However, sometimes, you may want to change things up. For example, maybe you want to send out a special St. Patrick's Day email that uses more green colors but you don't want to affect your automations. To accomplish that, you can change the theme to be Not Shared for a particular email. Under the Actions menu in the bottom-left corner of the editor, click on the Change Theme action.
You will then get to change between Shared and Not Shared. After selecting the preferred theme, click the Save button. The Save button will not be active unless you have made a change.
When not shared, changes to the header, footer, and theme settings are isolated to that particular email and you can craft the luckiest St. Patrick's Day promotion without the design of any of your other emails changing. You can do this to both newsletters and automations. You can even change newsletter templates (found on the Content page) to not use the shared theme. If you change a template to Not Shared then any newsletters you later create based on that template will also be set to Not Shared automatically.
When going from Not Shared back to Shared, any changes made to the header, footer, or theme settings will be completely lost as the shared settings will replace them.