Problem with background image for navigation menu items

Hello there,

I have a problem which needs urgent solution so please help!
I am customizing the header and the navigation bars quite a lot and I need to use a background image for the menu items. The image is set but the problem is that I can’t change anything like background-size or background-position. I tried in 2 ways: with a child theme and by adding custom css in Custom includes -> Custom style. If I inspect in the browser the custom css appears for the rule that I created (nav#nav-top ul li.menu-item) which is stronger than any other rules. Also I tried to add style on the element in firebug but still no effect. The only thing that works is when I add the custom css to a rule which comes from /uploads/suffusion/custom-styles.css (#nav-top ul li, #nav-top.continuous ul li, #nav-top ul ul li, #nav-top.continuous ul ul li, #nav-top ul li a, #nav-top.continuous ul li a, #nav-top a.current li a, #nav-top.continuous a.current li a). Any css added here has effect but it’s crazy that not even the style added on element is not working and I don’t know why or how to solve this.
So please help!