zulip/static/styles/user_circles.css

.user_circle_green,
.user_circle_orange,
.user_circle_empty,
.user_circle_empty_line,
.user_circle_fraction {
    border-radius: 50%;
    border: 1px solid;
    float: left;
    position: relative;
    top: 5px;
}

.user_circle_green {
    background-color: hsl(106, 74%, 44%);
}

.user_circle_green,
.user_circle_fraction {
    border-color: hsl(106, 74%, 44%);
}

.user_circle_orange {
    border-color: hsl(29, 84%, 51%);
    background: linear-gradient(
        to bottom,
        hsla(0, 0%, 100%, 0) 50%,
        hsla(29, 84%, 51%, 1) 50%
    );
}

.user_circle_empty {
    background-color: none;
    border-color: hsl(0, 0%, 50%);
}

.user_circle_empty_line {
    border-color: hsl(0, 0%, 50%);

    &::after {
        content: "";
        background: hsl(0, 0%, 50%);
        height: 1.5px; /* 1px is too thin, 2px is too thick */
        width: 6px;
        display: block;
        margin: 3.25px auto 0; /* (total height - line height) / 2 = 3.25px */
    }
}