I needed to add a bottom border to a sticky header. There’s no way I could do this through the Skins section. Perusing the generated code revealed nav.title-header was the style used, but it was not accessible via the internal or external editors for any of the stylesheets. However, it can be done manually.
Add this line of code to the main stylesheet which is named MainStyles.css or Styles.css located in the Resources > Stylesheets menu.
Right-click the stylesheet. Select Open with > Internal Text Editor. Insert:
border-bottom: 2px solid #127fc2;
This will add a 2 pixel solid border with color rgb(18, 127, 194). Change as needed.