<a class="theme-button decor_btn" href="#">Обратный звонок</a>

.theme-button{
    text-align: center;
    background: #DE9F8E;
    color: #fff;
    font-weight: bold;
    font-size: 14px;
    line-height: 18px;
    letter-spacing: .75px;
    display: inline-block;
    width: auto;
    text-transform: uppercase;
    margin: 0px auto;
    padding: 15px 21px;
    white-space: nowrap;
    text-decoration: none;
    border-radius: 30px;
  position:relative;
}

.decor_btn::before{
    content: '';
    position: absolute;
    border-radius: 30px;
    left:0;
    top:5px;
    left:5px;
    right: 5px;
    bottom: 5px;
    background-image: url("data:image/svg+xml;utf8,<svg width='100%' height='100%' xmlns='http://www.w3.org/2000/svg'><rect width='100%' height='100%' rx='20' style='fill: none; stroke: white; stroke-width: 2; stroke-dasharray:7 5'/></svg>");
}