<button class="buttonclass"><i class="iconclass plus-icon"></i></button>

.buttonclass {
  width: 30px;
  height: 30px;
  text-align: center;
  position: relative;
  border-radius: 20px;
  background-color: #1DBE60
}

.iconclass {
  width: 20px;
  height: 20px;
  margin: 7.5px;
}

.buttonclass .iconclass {
  margin: 0;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

.plus-icon {
  content: url(http://uxrepo.com/static/icon-sets/mfg-labs/svg/plus.svg);
}

.plus-icon {
    -webkit-mask: url('../images/plus.svg') no-repeat 50% 50%;
    mask: url('../images/plus.svg') no-repeat 50% 50%;
    -webkit-mask-size: cover;
    mask-size: cover;
}

.white {
    background-color: rgb(255,255,255);
}