html, body {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  background: #323232;
  color: #111;
  font-weight: 300;
  font-size: 14px;
  font-family: -apple-system, "PingFang SC", "Source Han Sans SC", "Noto Sans CJK SC", sans-serif;
  line-height: 1.8;
}

p {
  margin: 0;
  padding: 0;
}

a {
  color: #fff;
  text-decoration: none;
}

img  {
  display: block;
  width: 100%;
  transition: transform 0.5s ease-in-out;
}

img:hover  {
  transform: scale(1.02);
}

.header {
  position: fixed;
  bottom: 0;
  z-index: 999;
  border-top: 1px solid #404040;
  background: #252525;
  width: 100%;
}

.back {
  display: block;
  float: left;
  font-size: 18px;
  line-height: 40px;
  margin: 20px 40px;
  opacity: 1;
}

.back:hover {
  opacity: 0.5;
  transition: 0.5s;
}

.copyright {
  display: block;
  float: right;
  font-size: 18px;
  line-height: 40px;
  margin: 20px 40px;
  opacity: 1;
}

.copyright:hover {
  opacity: 0.5;
  transition: 0.5s;
}

.cases {
  display: block;
  float: left;
  width: 25%;
  overflow: hidden;
  opacity: 1;
}

.cases:hover {
  opacity: 0.8;
  transition: 1s;
}

.gototop {
  position: fixed;
  right: 20px;
  bottom: 100px;
  z-index: 999;
}

.gototop a:hover {
  border-bottom: 0;
}

.gototop-button {
  display: table;
  width: 50px;
  height: 50px;
  border-radius: 25px;
  background: rgba(0, 0, 0, 0.1);
  color: #fff;
  font-size: 30px;
  line-height: 50px;
  text-align: center;
}

.gototop-button:hover {
  background: rgba(0, 0, 0, 0.2);
  transition: 0.3s;
}

.clearfix {
  clear: both;
  float: none;
}

@media (max-width: 999px) {
  .cases { width: 50%; }
}

@media (max-width: 666px) {
  .back { font-size: 16px; margin: 10px 40px; }
  .copyright { font-size: 16px; margin: 10px 40px; }
  .cases { width: 100%; }
}