This is code to create a CSS class that can be applied to an element so the links within are white.
CSS
.white-link a {
color: white;
}Paste that into the Site Editor > Styles > Additional CSS box.
Next, go to the element you want to style. I did this for WPUser’s Login/Out link, which is in a Navigation element. I selected the link, and then Advanced (at the bottom of the block properties), and added “white-link” to the Additional CSS Classes box.
Note that there’s no “.” before “white-link” in the Additional CSS Classes box.
