<style>

.bg .tn-atom::before {
    content: "";
    position: absolute;
    top: -2px;
    left: -2px;
    right: -2px;
    bottom: -2px;
    background: linear-gradient(45deg, #fe2858, #2af0ea);
    z-index: -50;
    box-sizing: border-box;
    border-radius: inherit;
}
.bg .tn-atom::after {
  content: '';
    position: absolute;
    top: 0px; right: 0px; bottom: 0px; left: 0px;
    border-radius: inherit; /* !importanté */
    background: #000;
}
</style>