To find and edit any color in the widget section, what I use is an extension call Firebug for Firefox. From there you can see the css code to fine each widget background-color. Now if the widget doesn’t have a background-color, that means it is using the body background-color.
For example: The welcome widget background-color is #303236 on or about line 1231.
Again, use firebug to fine the widget’s name and change the background-color.