Update: Bootstrap 5 now includes opacity utility classes. Below are custom styles you can use in older versions.
Bootstrap 4 does not have any opacity utility classes. You can add this to a custom stylesheet to quickly add the ability to write .opacity-(0-5) to change the opacity of an element.
0= opacity: 0;1= opacity: 0.2;2= opacity: 0.4;3= opacity: 0.6;4= opacity: 0.8;5= opacity: 1;