discourse/app/assets/stylesheets/mobile/banner.scss

// --------------------------------------------------
// Banner
// --------------------------------------------------

#banner {
  // go full width on mobile, by extending into the 10px wrap
  // borders on left and right
  margin: 0 -10px;
  max-height: 180px;
  @media all and (max-height: 499px) {
    max-height: 100px;
  }
}