You were right in assuming that such a plugin may not exist.
Since each theme’s structure can widely change, there would be no reliable way for a plugin to handle that kind of a task.
There is one way you can simplify things assuming you have Firefox installed:
Firebug with a color picker.
You will be using Firebug to figure out what class you actually want to edit and the color picker to help you select the colors that you want.
With Firebug, you can make the changes as you are looking at your page before you decide to make it permanent by modifying the theme’s files.
You can find Firebug here:
https://addons.mozilla.org/en-US/firefox/addon/1843
If you are not using Firefox, I can’t help you there.
(Many stealth edits.)