Install Sections
Install on multiple themes
Use one section across every theme in your store, including drafts.
Install the same section again
Go to My Sections.
Click Add To Theme on the same section again.
Under Choose Theme, pick a different theme.
Click Add to theme.
Repeat for each theme you want the section in.
Test on a draft theme first
In the Choose Theme dropdown, pick a theme that is not marked (MAIN) — every theme in your store is listed, published or not.
Install the section there and style it in the theme editor.
Publish that theme when you are happy with it.
No extra cost
You pay for a section once. Installing it on more themes is always free.