Menu isn't sticky!

I’m not sure if you should remove those instances of “!important” straight away. They’re put there for a reason (to override other styles) by the theme creator, so removing them may break the general theme styles. As a general rule, you should never just change the theme’s style.css to begin with.

However, I do think that these specific styles are indeed only used when you don’t have the full width format. My guess is that when you do use full width, there are no styles (that relate to the navigation) that use the “!important” tag and yes, you’re right, the “offending” CSS only kicks in when you’re using this box format.