Add a simple border to the top and bottom of the site header. This plugin uses css editor and works with Squarespace 7.1.

From your Squarespace account, go to the Custom CSS Editor. Copy and paste the code below into the Custom CSS Editor box.

Adjust both border px and color values to your liking.

You can also adjust the margin and width values to control the outer spacing around the header and borders.

Note: This code also applies to the header in mobile.