On pages where you have a header hero area (slider or image), the menu is transparent when the site is not scrolled. You can use the following code to add a background color to it:
.site-header { background-color: rgba(72, 181, 206, 0.6); }
Tip: the color code above includes a bit of transparency. You can use a solid color instead if you want (like #333).
INFO: This is a CSS snippet. If you don’t have a child theme or don’t know how to use one, we recommend that you add this snippet in Customize > Additional CSS.