Stationary is now back in Yahoo Mail Beta, which was missing the feature, and can be found in the list of apps available to all users. Another tool you could use to spice up your messages is Stationery, which enabled users to add custom background images to emails.
According to Miriam Geller(Yahoo Mail Product Director) on yahoo blog:
With the latest release of Yahoo! Mail Beta, one new feature that is now included is the ability to add stationery (background colors and image templates) to your messages. Stationery is a feature that existed in the previous version of Yahoo! Mail. Since the release of Yahoo! Mail Beta, we have gotten feedback from customers asking us to bring this feature back. We listened and are happy to report that with the help of a 3rd party developer and our Yahoo! Mail Development Platform, we were able to do just that.
How to use Stationery:
1- Go to the Applications area of Yahoo! Mail and click on the application named “Stationery.”
2- Once you click on Stationery you can then choose the background you wish to use in your message. Select from the drop-down menu and click on the background image of choice.
3- After clicking the background image, a new message window is launched. From there, simply type your message and send it along with your favorite background applied.