style.css

:root {
  --base-color: #333333;
  --back-color: #f7f7f7;
}

body {
  font-size: 1.5rem;
  color: var(--base-color);
  background-color: var(--back-color);

  padding: 0px 30px;
}

a {
  text-decoration: none;
  color: var(--base-color);
}
a:hover {
  text-decoration: underline;
}

/* header */
.header > .title::before {
  content: "#";
  margin-right: 10px;
}

/* main */
.collection {
  width: 100%;
  display: flex;
  flex-wrap: wrap;
}

.category > .title::before {
  content: "##";
  margin-right: 10px;
}
.category > .title::after {
  display: block;
  content: "";
  height: 2px;
  background-color: var(--base-color);
}
.category + .category {
  margin-top: 100px;
}

.card {
  width: 320px;
  box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.2);
  transition: 0.3s;
  margin: 20px;
}
.card:hover {
  box-shadow: 0 12px 24px 0 rgba(0, 0, 0, 0.2);
}

.card .title {
  text-align: center;
  margin: 20px 0;
}

.iframe-wrapper {
  width: 320px;
  height: 240px;
}
.iframe-wrapper .iframe {
  width: calc(320px * 1.5);
  height: calc(240px * 1.5);
  transform-origin: 0 0;
  /* original / iframe */
  transform: scale(0.666);
  border: none;
}

/* footer */
.footer {
  text-align: center;
}

/* GitHub Corner */
.github-corner svg {
  fill: #151513;
  color: #fff;
  position: fixed;
  top: 0;
  border: 0;
  right: 0;
}
.github-corner:hover .octo-arm {
  animation: octocat-wave 560ms ease-in-out;
}

@keyframes octocat-wave {
  0%,
  100% {
    transform: rotate(0);
  }
  20%,
  60% {
    transform: rotate(-25deg);
  }
  40%,
  80% {
    transform: rotate(10deg);
  }
}

@media (max-width: 500px) {
  body {
    font-size: 1rem;
    padding: 10px;
  }
  .collection {
    justify-content: center;
  }

  .github-corner:hover .octo-arm {
    animation: none;
  }
  .github-corner .octo-arm {
    animation: octocat-wave 560ms ease-in-out;
  }
}