Twenty Eleven gradiant colour header menu

I have created my website on twenty eleven. Very good theme to start with and serves its purpose very well. I am struggling to find out how to make the header menu bar and the hover colors gradiant. I saw a youtube clip online showing a plugin called theme colors, but I do not have this as part of the theme nor can I find the plugin. Within this theme, you can set the gradient. Has anyone managed to successfully set a gradient colour in this theme?