How to change the border color below the navigation and logo area

The top bar background color setting adjusts the top announcement bar's background color and also the border around the meganav, drop-down menu and meganav column headings:

Example, changing the color to a red value, you'll easily see all three elements:

Example with a drop-down menu:

The account and search icons will pick up the color from the top bar text setting. This gets changed after you Save the settings: