Change Top Menu Bar Opacity

.site-header-topbar {
    background-color: rgba(255,255,255,0.75);
}

To add CSS: If your theme has a custom CSS option, use that to add the CSS shown above. If not, install the plugin Simple Custom CSS. Or, if you have Jetpack installed, enable its Custom CSS module.

Learn the Chrome Developer Tools to help you see and test changes to your CSS.