Hi,
Do you have a link to your site? Or is this on a local install?
If you’re using the latest version, then you can find the CSS for it around line 1007 in the style.css file –
/* Site Header
---------------------------------------------------------------------------- */
.site-header {
background-color: #000;
box-shadow: 0 0 20px rgba(0, 0, 0, 0.05);
color: #fff;
padding: 0 40px;
}
If you don’t want to edit the css file, you can keep your Custom CSS separate by copying the code above (just the .site-header code) and adding it to the “Additional CSS” page located in the Customizer….. Appearance > Customize
Let me know if that helps