simple-modal.css

.modal, .modal * {
  -webkit-box-sizing: border-box;
          box-sizing: border-box;
}

.overflow-hidden {
  overflow: hidden;
}
.overflow-hidden body {
  overflow: hidden;
}

.modal {
  position: fixed;
  left: 0;
  right: 0;
  width: 100%;
  height: 100%;
  top: 0;
  bottom: 0;
  background-color: rgba(0, 0, 0, 0.5);
  z-index: 20;
  opacity: 0;
  -webkit-box-sizing: border-box;
          box-sizing: border-box;
  -webkit-transition: opacity 0.3s ease-in-out;
  transition: opacity 0.3s ease-in-out;
}
.modal[aria-hidden=true] {
  display: none;
}
.modal[aria-hidden=false] {
  display: block;
}
.modal.is-open {
  opacity: 1;
}
.modal__wrapper {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  padding: 20px 20px;
  max-height: 92vh;
  height: 100%;
  width: 100%;
}
.modal__inner {
  position: relative;
  max-width: 800px;
  margin: auto;
  background-color: #fff;
  padding: 40px 30px;
  border-radius: 12px;
  max-height: 100%;
  overflow: auto;
}
.modal__close {
  position: absolute;
  cursor: pointer;
  right: 5px;
  top: 5px;
  font-size: 40px;
  border: none;
  background: 0;
  outline: transparent;
  -webkit-box-shadow: none;
          box-shadow: none;
}