I installed this plugin on a test site and it doesn't seem to be fully compatible with Twenty Eleven theme, therefore I don't recommend using it.
If you only want to edit the CSS of your menu and sidebar, you can easily create a child theme of Twenty Eleven like this:
1. Create a folder in your themes folder from your server (/wp-content/themes/) called let's say my-twentyeleven (don't use spaces).
2. Create a file that you call styles.css with the following code inside:
Theme Name: My Twenty Eleven
Text Domain: twentyeleven
/* You must first include the original css from the parent theme */
3. If you go Appearance->Themes in the WP dashboard you'll see a new theme called My Twenty Eleven by jojforit. Activate it.
4. Now you can add your own CSS lines that will overwrite the lines from the original Twenty Eleven theme and they won't be lost when you update the original theme. For example, to change the background color of the menu you can add a line like this in the style.css file that you've just created: