.theme-btn {
     position: relative;
    z-index: 2;
    overflow: hidden;
    vertical-align: middle;
    display: inline-block;
    border: none;
    text-transform: uppercase;
    text-align: center;
    background-color: #ffc734;
    color: #000000;
    font-size: 18px;
    font-weight: 600;
    line-height: 1;
    padding: 15px 30px;
    min-width: 170px; 
}

.theme-btn:before {
    content: "";
    position: absolute;
    height: 100%;
    bottom: 0;
    left: 0;
    width: 0;
    background-color:#000000;
    z-index: -1;
    transition: all 0.4s ease-out;
    border-radius: inherit;
}

span {
    margin: 0px;
    transition: all 0.4s ease-in-out;
}

.theme-btn:hover {
    color: #fff;
    box-shadow: none;
}

.theme-btn:hover::before {
    width: 100%;
}