When scrolling, make desktop navigation bar change from transparent to solid background

Site URL: http://www.tnvirtuallaw.com

I used to have this set up properly, but not sure what changed. 

Currently, my navigation bar color is set to "transparent," so the background is the city image on the homepage. When scrolling, the navigation bar used to change to a solid dark grey background (leaving the white logo and white page links visible wherever you are on the website). Now, when you scroll, it remains a transparent background, making it harder to see. I never had a code injection before for this, so I'm pretty sure it is an option without adding new code. 

Any help is appreciated. 

Add to Home > Design > Custom CSS

/* on scroll header color */
.header-announcement-bar-wrapper.black.shrink {
    background: grey;
}