Yes, if you want HE icons, it needs to be like this:
.he-circle-down:before {
content: "\ea0b";
color: rgba(0,0,0,1);
font-size: 25px;
}
At a high level, .he-circle-down:before is the original icon, your referencing it after you have hidden it, hence the :before. content: "\ea0b" is the HE icon you want to display.
To see a full write-up head over to this link where I shared several different modifications to HE dashboard tiles using CSS.