A small green line in the Shopify Impulse Header

[sim25ecom]

Hi I would like to add a green, small line under the menu items of the shopify impulse theme like in the screenshot attached. 

color: #83861B

2px

full width (desktop & mobile)

There must be white space under the line like on the screenshot image.

I am grateful for any help. Thanks in advance!

[Ben310]

I have just one question - why? Will it really improve the UX? It could even take your Impulse theme out of the updates stream. And if your menu adds items or becomes longer or is on a smaller screen size then it will do a line break and look totally NAF!