This is the same process as Creating a Public Collection but with an extra step to turn it into a theme (I.e Step 4).
Go to My Collections in the top navigation
Type the collection name under ‘Create New Collection’ and click ‘Create’
Beside ‘Access’ choose ‘Public’ from the dropdown menu
Select the Theme category that the collection belongs to. If there is no appropriate theme category, enter a new theme name.
Click Save