(Theme: Colormag) Nav menu button question…

Hi,

I’m hoping someone can help me to change the appearance of the navigation menu ‘button’, that appears on mobile devices when the nav menu won’t fit on the screen.

It appears as three white lines, in the top left hand side of the screen.

I would like to know how to change this symbol/icon to one of my own?
And how to change the width and height of the button/icon?

All help will be much appreciated.

Look forward to your replies.

Thanks,

Danny.