header {
  /* Change this color to anything you want! */
  /*   * background: #2c3e50; /* A dark blue/gray */
  background: linear-gradient(135deg, #3B82F6, #10B981);
  padding: 10px 20px;
  text-align: left; /* Or center, or right */
}

/* linear-gradient(135deg, #3B82F6, #10B981); */
/* Make sure the logo image itself has a transparent background */
header img {
  /* This style isn't strictly needed if the image is transparent,
     but it does no harm. */
  background-color: transparent;
}