Adding custom css in a menu link – facing difficulties.

I am adding a link in navigation menu. I want its background color to be different from others but default color is overriding it. Custom color is showing but its is behind default color [noticed when increased padding of custom css].

How can I fix it?

also would like to known is “!important;” just a caution or do some work in theme (like overriding other coding).

I am using twenty twelve theme.

Thanks!