There are a couple of things in could be.
Your selector could be wrong, or not specific enough. If you copied the exact selector, double check that you copied the exact selector they used.
The order of your stylesheets could be wrong. Make sure that jquery.ui.css and theme files are getting loaded before yours.
It could be something else. I find the best way to debug things like this is in the styles panel in Chrome Developer Tools. Go to the tab that says computed; choose width; and click where it says width, it will bring you to the selector that created that rule. Uncheck the width rule and then see what width rule gets applied. Eventually you will uncheck all the applied width rules or find out which one is overriding yours.
You may also discover something else like - your rule isn't loaded at all. Maybe you are using a cached version of your stylesheet, etc.
This rule is for sure overridable - there is either a logic error or a typo somewhere, and you just need to find it.