You can use the :before attributes of CSS.
Didn't spend a lot of time on this selector, so you may be able to optimize it, but it should work for them:
#header-secondary > ul > li.item-logIn > button[title="Log In"]:before {
font-family: FontAwesome;
font-size: 30px;
display: inline-block;
content: '\f137';
}
#header-secondary > ul > li.item-logIn > button[title="Sign Up"]:before {
font-family: FontAwesome;
font-size: 30px;
display: inline-block;
content: '\f137';
}
