If you want to remove the opacity or transparency from the sticky navigation bar, just navigate to Theme Options -> General -> Additional CSS and copy/paste this code and save changes.
background: rgba(252, 252, 252, 1) !important;
You could also manipulate the opacity by altering the value “1” at the end of the CSS statement. Accepted values are in range from 0 – 1 and 0.5 represents 50% for example.
Additionally, if you do not want white sticky header, you can alter the color using first 3 rgba values.