#menu-main-menu is a <ul> within an HTML table-cell. The flex-box is a bit silly where it is located. If you want to center the menu, then center the table. These resets for the table and related elements are from the http://37.60.239.80/~dvkapuser/wp-content/plugins/dropdown-menu-widget/css/shailan-dropdown.min.css
so you should override the table in your local stylesheet. I doubt that you need to override anything other than the table. Start with the least (table only) and add more if needed.
FYI: You can add outlines around the contents of the menu boxes to see where they are and how they are behaving by doing something like this temporarily: