Just go to Gmail's Themes page, go to the "custom themes" section and pick "share your theme" if you already use "custom light" or "custom dark". If you don't use them, click "custom light"or "custom dark" and select a background image.
You can share your theme by email, using Google+ or you can simply copy the URL generated by Gmail. Here's an example.
When someone clicks the URL, Gmail will show a "theme change confirmation" dialog.