#header {
  display: flex;
  align-items: center;      /* vertical center */
  height: 80px;
  padding: 0 20px;
  background: none;         /* remove existing background if any */
  box-sizing: border-box;
  position: relative;       /* to enable z-index on children */
}

/* Logo inside the <a> */
#header > a {
  flex-shrink: 0;           /* prevent shrinking */
  margin-right: 20px;       /* spacing from banner */
  position: relative;
  z-index: 2;               /* logo on top */
}

#header > a > img#logo {
  max-height: 60px;
  height: auto;
  width: auto;
  display: block;
}

/* Banner as ::after pseudo element */
#header::after {
  content: "";
  flex-grow: 1;             /* fill remaining space */
  height: 60px;
  background: url('/filesender/images/banner.png') no-repeat left center;
  background-size: contain;
  display: block;
  position: relative;
  z-index: 1;
}