How to use icons from font awesome?

I tried to implement another bullet icon from Font Awesome, and did it this way:
a) I searched for an icon and found this one: check-square.
(The f-number is the same as in the icon as solid – I wonder how they are distiguished …)
b) I copied the following code from the original style.css line 2868, changing only the f-number and giving it a new class name, and put it in my child style.css, somewhere in chapter 11.1:

ul.list-icon-aufgabe li:before { 
	color: #6ba420;
	content: "\f14a"; 
	font-size: 20px;
	font-size: 1.25rem;
	padding-right: 10px;
	padding-right: 0.625rem;
	vertical-align: text-bottom;
}

c) Then I tested it in this page (scroll down). But it doesn’t work, yet it still shows the standard bullets and some error icons …
What am I doing wrong?