Change Background Colors via CSS

There is not currently a way to change the background color of a theme directly within the editor.  The only way to make this change is via the CSS of the theme.     

To change the background colors of a theme, go to the Design Tab and click the Edit HTML / CSS button. 

Then find the Body area of the CSS.  This is generally near the top of the page. 

Delete the line of "background:" code from what we see above and replace it with this line:
  
background: #000000;   

This will turn the background of the theme black.  You can change that to whatever other color you want using a different six-digit HTML Color Code.  

If you don't know what HTML Color Code you want to use, see the W3C Schools HTML Color Names page to find a color. 

After you've made the change, you'll see an immediate preview of this change in the preview window. 

If you like it, Save your changes.  You'll be asked to give your altered theme a name if this is the first time you've changed it. 

And there you go.  You will now have a different background color. 

If you want to get fancier, you can replace the background with an image instead of a color (this is more advanced than replacing with a color).  To do so, change the background line so it looks like this:

background: #000000 url( ) no-repeat;

Then upload your background image via the Files column on the left side. 

Once the image is uploaded, type the full name of the image within the parentheses.  Include the file extension (.jpg, .png, etc), too. 
  
background: #000000 url(fullnameofimage.png) no-repeat;

If you uploaded a small image and want it to repeat across the entire background, write it like this: 
  
background: #000000 url(fullnameofimage.png) repeat-x;

If you uploaded a large image and only want it to show in the background once, write it like this: 

background: #000000 url(fullnameofimage.png) no-repeat;