Hi, for who want a softer effect I share my CSS edit.
For profile page and post:

CSS Code:
/* Animated online indicator post and profile */
/* Online icon (fa-circle) */
.UserCard-lastSeen.online .fa-circle,
.PostUser .UserOnline .fa-circle {
color: var(--online-user-circle-color);
position: relative;
}
/* Pulse */
.UserCard-lastSeen.online .fa-circle::after,
.PostUser .UserOnline .fa-circle::after {
content: "";
position: absolute;
width: 10px;
height: 10px;
top: 1px;
left: 1px;
border-radius: 50%;
background-color: var(--online-user-circle-color);
box-shadow: 0 0 0 0 var(--online-user-circle-color);
animation: Flarum-Online-Pulse 3s infinite ease-out;
}
/* Animate pulse */
@keyframes Flarum-Online-Pulse {
0% {
box-shadow: 0 0 0 0 rgba(0, 200, 0, 0.6);
}
70% {
box-shadow: 0 0 0 10px rgba(0, 200, 0, 0);
}
100% {
box-shadow: 0 0 0 0 rgba(0, 200, 0, 0);
}
}
For Afrux online user:

CSS code:
/* Afrox online user list animation */
.Afrux-OnlineUsersWidget-users-item::after {
content: "";
position: absolute;
width: 4px;
height: 4px;
margin-top: -1px;
margin-left: -35px;
border-radius: 50%;
background-color: var(--online-user-circle-color);
/* initial alone */
box-shadow: 0 0 0 0 var(--online-user-circle-color);
animation: Flarum-Online-Dot 3s infinite ease-out;
}
@keyframes Flarum-Online-Dot {
0% {
box-shadow: 0 0 0 0 rgba(0, 200, 0, 0.6);
}
70% {
box-shadow: 0 0 0 6px rgba(0, 200, 0, 0);
}
100% {
box-shadow: 0 0 0 0 rgba(0, 200, 0, 0);
}
}