Help with mobile menu – toggle background color

I have a background image starting in the main menu(fades in from top). Looks good on desktop. On mobile it doesn’t look good as OceanWP seems to assume there will be a flat color for the site header so the background color is set to transparent.
What I’m trying to achieve. On mobile menu click -> toggle white background color on site-header

I would need to toggle the following css on menu click to off or white. Anyone willing to help? My JS/jQuery skills aren’t good.

#site-header.transparent-header {
background-color: transparent;}

What its now:
https://www.dropbox.com/s/kadmf7ubtsof0eo/how_it_is_default_mobile.JPG?dl=0
What I’m trying to achieve:
https://www.dropbox.com/s/jmgsg1oo3zsezb8/how_I_want_it_mobile.JPG?dl=0