Unable to Modify Stylesheet in Editor

Hi i’m currently developing a website for my client. i’m having a problem modifying the css code of my site, when I make some change like setting my width from 900px to 960px it doesn’t change anything when i update my editor and view my site. I tried to change the background color of my menu and no luck. i clear my chache, Deactivate all my plugins but stil not working. i really need to start working this site. My theme is twenty ten. I prefer to use that theme as it is easy to use for me. I really need your help. Thank you.