Centering sub menu

The theme I am working with drops down a normal square sub menu, but I wanted to extend the width to be responsive to 100% width and display the sub menu items in a line, rather than listed vertically.

Ive been trying to troubleshoot this for a couple hours now and cannot figure it out.
I can get it to display in a line, and extend to its own width, but not to 100%, and I also cant get it to center.

Any help is greatly appreciated.
Thank you!