Code for line under header on mobile?

Site URL: https://www.efficientcontractorsinc.com/home

Hi 😀

I'm building a website on the Brine template. The website is still under construction. I am using this css code on a website to get a red line under the header, and it works great on desk top and tablet, but it doesn't appear on mobile (and I want it too). Can anyone help?

.Header {
border-bottom
  : 4px solid #BB0000 !important;
}

Password is abcd

Use this code for mobile

.Mobile-bar.Mobile-bar--top {
    border-bottom: 3px solid red;
}