Overwritten TwentyEleven–Child Theme?

Yes you want to make a Child Theme for sure. Luckily it is pretty easy if all you changed was the css.

Basically create a folder on your desktop (YourThemeName). Inside of it create a file called style.css then using a text editor or whatever web editor you may use (Dreamweaver, etc) put this at the top of style.css

/*
Theme Name:     Twenty Eleven Child
Theme URI:      http://example.com/
Description:    Child theme for the Twenty Eleven theme
Author:         Your name here
Author URI:     http://example.com/about/
Template:       twentyeleven
Version:        0.1.0
*/
@import url("../twentyeleven/style.css");

You can edit most of those details but you want to keep the Template and @import url the same.

Next up add any of the changes you made to the 2011 css (you only need to add the changes since the @import function will bring it the rest). Anything you do in this file will overwrite the Parent’s style.

Now zip the folder and upload it via the Admin-Appearance-Themes

I hope that made sense, here is the codex page if you need more help.
http://codex.wordpress.org/Child_Themes

Good stuff! Happy coding!