Move main menu to top of screen

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This is a two step solution:

  1. Install plugin “Options for Twenty Seventeen” (Link)
  2. Add the following CSS to “Design” > “Customizer” > “Additional CSS”
.navigation-top {
    position: fixed;
    bottom: auto;
    left: 0;
    right: 0;
    top: 0;
    width: 100%;
}

.custom-header {
    padding-top: 50px;
}