Border Removal

I am currentl working on my website and would want some help as it relates to removing the black border line that shows in the mobile background menu. when you reset the all design changes it does not show as the default color is black. When you change the color you can see the line around the menu border, search button if on and also the menu icon. I would like to remove this but it is not working using the the basic css settings in the design area of the theme.