Help with Main Menu CSS Formatting, Responsive Full Width

I am building a “responsive” web site based on the Enfold theme.

The main menu does not stretch the full width of the container, rather, it is aligned to the left.

https://ncsera.com/the-7-step-planning-process/

I would like the menu buttons to stretch 100% across the container, and then let the cell padding shrink and expand along with the screen/device size.

Notice at the bottom of the blog post there is a “share this entry” feature that is built that way. The bar stays 100% width while the size of the individual buttons shrink and expand.

How difficult would it be to make my “main menu” do the same thing?

Thanks for any help or pointing me in the right direction!!