body {
  font-family: 'Segoe UI', sans-serif;
  margin: 0;
  padding: 0;
  background: #eef1f5;
  color: #333;
}
header {
  background: #5c6bc0;
  color: white;
  padding: 2em;
  text-align: center;
}
main {
  padding: 2em;
}
section {
  background: #fff;
  margin-bottom: 2em;
  padding: 2em;
  border-radius: 10px;
  box-shadow: 0 2px 10px rgba(0,0,0,0.1);
}
section h2 {
  color: #3f51b5;
}
.intro {
  font-weight: bold;
  margin-bottom: 1em;
}
ul {
  margin-top: 1em;
}
li {
  margin-bottom: 0.5em;
}
.mod-img {
  width: 100%;
  max-width: 600px;
  display: block;
  margin: 1em auto;
  border-radius: 8px;
}
.actividad {
  background: #e3f2fd;
  padding: 1em;
  border-left: 5px solid #2196f3;
  margin-top: 1em;
  border-radius: 5px;
}
.video {
  margin-top: 1em;
}
.iframe-container {
  position: relative;
  width: 100%;
  padding-bottom: 56.25%;
  height: 0;
  overflow: hidden;
}
.iframe-container iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}
footer {
  background: #3f51b5;
  color: white;
  text-align: center;
  padding: 1em;
}
a {
  color: #303f9f;
  text-decoration: none;
}
a:hover {
  text-decoration: underline;
}