I did the following with the browsers' developer tools and got the submenu centered.
Delete in your CSS position: relative
on #main-nav .vnav__item
and on .vnav__item
.
The other thing to do is to change in the CSS on #main-nav .vnav--horizontal .vnav__subnav
the property left: 370px
to left: 0
.
If you add width: 100%
to #main-nav .vnav
and delete the left & right padding of 15px on .navbar-collapse
, then the submenu expands to the full width of the layout.
On a side note... why are you editing and changing stuff on a site that is live? Better to make a copy of the site and tweak stuff in there.