Default menu width is too small

I would like to make the submenu bigger by percetage, but the initial width of the menu itself is too small (the width of the menu is only as big as the menu items, it doesn’t extend to take up all the width of the screen (stretch)) . How can I make the menu bar wider?

Also, when I try to play with the outer and inner width of the submenu, only the “body” works, and still then the submenu items are not centered but they are all on the left side. How can I resolve this? Thank you