The Blog Theme Editor
Editing a blog theme
You should see something like this:
Then click on the name or Clone of the themes you want to edit or add your external theme.
You can edit the name of your new theme by clicking Edit in the top-right corner and typing the desired name into the text field.
Dynamic parts of a template
To use the new theme for a podcast, go to the Blog tab in the podcast settings and select the newly created theme.