I have had the background and the format of my emails on my computer screen go to a format that stinks.
How can I get it to go back to the old format?

I have some bad news for you:
Yahoo Shuts Down Mail Classic, Forces Switch To New Version That Scans Your Emails To Target Ads | TechCrunch (http://techcrunch.com/2013/06/02/yahoo-shuts-down-mail-classic/)

I too have Yahoo and was horrified to see what they had done to my email formatting. I Googled how to change it back to Classic. At the top right of the screen is a little cog or wheel. Click on that. You'll see a formatting choice to click on for Classic (it's called Basic). If you can't find it, post again. It's not quite the same as before, but close enough so I can stop screaming.

Wheel -> Settings -> Viewing email -> Basic