Stretch nav menu to width of main content

Hi.

I’ve changed the background color of my navigation menu (.main-navigation) but I’ve noticed that the bar doesn’t stretch the full width of the main content area. Is there a way to have it so that the background of the menu stretches to the edge of the main content area?

Also, what properties do I need to change to change the background color of child menus? I’ve only been able to change the background color of the parent link in the menu.

I would also like to change the colors of buttons, such as the Search button. Can someone please tell me how to do this?

Thank you.