Can fix with some CSS
Create a new theme-component add this to common css
// Who's Online Customization
.discovery-list-container-top-outlet.online_users_widget {
margin-top: 0.75em;
margin-bottom: 0.75em;
padding-top: 0.5em
padding-bottom: 0.5em;
}
You can also add custom background color, borders etc…