change .tc-header for mobile only

I have added an iframe to a client’s website to display their blog. It works great, but I realize I need to remove an image I am using in the header in only the mobile version of the css. The darker grey with a lighter grey beneath does not look as good on mobile as it does on the larger display.

I tried putting @media screen .tc-header in my child theme css then removing the background image but nothing happened. I am sure I am doing something simple wrong. Help!

The main website is: https://www.ctgoldandsilver.com/
(It is NOT php and I did not design it, the client wishes to keep it as is for now)
The blog I created for them is at:https://www.ctgoldandsilver.com/blog/

Any and all help appreciated.