Adding coloured boxes behind Navigation Links

Site URL: https://plum-ellipsoid-y6lj.squarespace.com/config/settings/site-visibility

My navigation bar looks like this.

I want to put just the text 'About Carlton' + 'Work' in a white box each with black type (almost like a button).

I can't for the life of my figure out how to adjust them (or add another button to my Navigation).

Please could I have some help!

Thanks!

@joeblackham1, you can use custom CSS to do that, however what you have above is a common design practice and helps draw emphasis to your call to action (the red contact link). 

Are you concerned about the visibility of the header links? If so an alternative is to add a subtle dark background to the entire navigation so that the white text stands out a bit more and doesn't get lost in the background image