How to remove/customise line in main header

From this

To this

Find this code by ctrl+f

ctrl+c to copy

#header-wrapper {

(this is for the line outside)

AND

#header { 

(this is for the line inside)

JUST

change the NUMBER to 0

border: NUMBER px

OR 

You can add more code to it.

border: 1px solid #ffffff;

red to change the thickness of the border

pink to change the style of the border

blue to change the colour of the border (you do not need to add this code if you want to)