Want to add a small/thin box or banner behind the text in the navigation instead of an underline

Wanted to add a box or banner behind the navigation links at the top of the webpage's, tried a couple different things but no idea how I would go about doing it.

Ive linked an image below for how I want it to look. if anyone could point me in the right direction would be much appreciated!

Thank you

If you have a green image, we can give you code to achieve this