CSS实现“圆角渐变色边框+内发光效果”的“透明”按钮

 这只是折中方案，因为并没有实现背景全透明的效果。

.btn{
    margin: 0 5px;
    padding: 5px 20px;
    cursor: pointer;
}

.btn::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    border: 1px solid transparent;
    background-clip: padding-box, border-box;
    background-origin: padding-box, border-box;
    background-image: linear-gradient(to right, #1a2324, #1a2324), linear-gradient(0deg, #a1ffd2, #a7cb50);
    box-shadow: 0 0 18px 0 rgba(46, 161, 108, 0.55) inset;
    border-radius: 20px;
    opacity: 0.4;
    z-index: -1;
}

效果如下：