Rather than putting your changes into nivo.css, I'd recommend you put your changes into a new CSS file, called for example "myoverrides.css". Add it to the bottom of the style.config file for your skin like this:
<file>myoverrides.css</file>
With your CSS file loading last, you'll be certain that those directives will take precedence over anything that was specified in a previous file.
Also, before you test CSS changes you should go to Administration > Advanced Tools > Design Tools > Cache Tool and click the "reset skin guid" button. That will trigger a reset so all browsers that visit your site will reload the CSS from scratch.
Jamie