Can CSS be over ridden on Android the same way userContent.css works on Firefox?
There is currently no way to apply a user stylesheet with the default Android browser.

If you Firefox isn't available on your phone, you could install the APK manually, but it may not work since it isn't supported.

You could download the Opera browser, connect it to remote debugging using the Dragonfly debugger, and then manually apply the stylesheet via debugging--but there are a lot of steps to that and you would have to add the stylesheet on every page load. This may not be a solution depending on what your needs are for the user stylesheet.