Mystique theme navigation bar

As you can see here in my blog I have moved the navigation bar so it can be stuck to the content box, but whenever I try to edit its color and turn it to black, nothing seems to change, I have tried of options using custom CSS and editing Stylesheet, but nothing changed since moving the navigation bar downwards.

Any CSS code I can use for the color and for rounded corners on the nav bar?

if not, any suggestions?