Instead of recreating the cool custom theme of another drawing, simply copy it to your new drawing by following this tech-recipe:
1. Open the drawing that has the custom theme.
2. Click a shape with the desired custom theme applied.
3. Click Edit and select Copy.
4. Open the drawing that you want to copy the custom theme to.
5. Click Edit and select Paste.
6. Delete the shape that you just pasted.
The custom theme is now added to the Themes task pane and ready to use.
