Thank you
Presuming you wish to add it to a H1 heading, try adding the following to the Custom CSS field of your templates parameters (Extensions -> Templates -> [YourTemplateStyle] -> Custom CSS)....
h1::before,
h1::after {
position: absolute;
bottom: 0;
left: 0;
height: 4px;
content: '';
opacity: .8;
}
h1::before {
background-color: rgba(0,0,0,0.25);
right: 0;
}
h1::after {
background-color: var(--primary);
right: calc(100% - 70px);
}
Ciaran