/* Universal Selector (*) */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

/* Type Selector */
body {
  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
  line-height: 1.6;
  padding: 2.5rem 1.25rem;
  background-color: #f5f5f5;
  color: #333;
  max-width: 75rem;
  margin: 0 auto;
}

/* Class Selector */
.section {
  margin: 1.875rem 0;
  padding: 1.5625rem;
  border-radius: 0.5rem;
  background-color: white;
  box-shadow: 0 0.125rem 0.25rem rgba(0, 0, 0, 0.1);
  transition: transform 0.2s ease;
}

.section:hover {
  transform: translateY(-0.125rem);
  box-shadow: 0 0.25rem 0.5rem rgba(0, 0, 0, 0.15);
}

/* ID Selector */
#header {
  background: linear-gradient(135deg, #2c3e50, #3498db);
  color: white;
  padding: 2.5rem 1.25rem;
  text-align: center;
  border-radius: 0.5rem;
  margin-bottom: 2.5rem;
}

/* Headings */
h1 {
  font-size: 2.5em;
  margin-bottom: 0.625rem;
  text-shadow: 0.125rem 0.125rem 0.25rem rgba(0, 0, 0, 0.2);
}

h2 {
  color: #2c3e50;
  margin-bottom: 0.9375rem;
  font-size: 1.8em;
}

/* Code blocks */
code {
  display: block;
  background-color: #f8f9fa;
  padding: 0.9375rem;
  border-radius: 0.375rem;
  border-left: 0.25rem solid #3498db;
  margin: 0.9375rem 0;
  font-family: "Courier New", Courier, monospace;
  font-size: 0.9em;
}

/* Links and buttons */
.back-btn {
  position: fixed;
  top: 1.25rem;
  left: 1.25rem;
  padding: 0.75rem 1.5rem;
  background-color: #2c3e50;
  color: white;
  text-decoration: none;
  border-radius: 1.5625rem;
  font-weight: 500;
  box-shadow: 0 0.125rem 0.25rem rgba(0, 0, 0, 0.2);
  transition: all 0.3s ease;
}

.back-btn:hover {
  background-color: #3498db;
  transform: translateY(-0.125rem);
  box-shadow: 0 0.25rem 0.5rem rgba(0, 0, 0, 0.2);
}

/* Paragraphs */
p {
  color: #555;
  margin-bottom: 0.9375rem;
  font-size: 1.1em;
}
.intro {
  margin-bottom: 0.9375rem;
  font-size: 1.1em;
  color: #f4f4f4f4;
}

/* Input fields */
input[type="text"] {
  padding: 0.625rem;
  border: 0.125rem solid #ddd;
  border-radius: 0.25rem;
  width: 100%;
  max-width: 18.75rem;
  transition: border-color 0.3s ease;
}

input[type="text"]:focus {
  border-color: #3498db;
  outline: none;
}

/* Special elements */
.child {
  padding: 0.9375rem;
  background-color: #e8f4f8;
  border-radius: 0.375rem;
}

.nested-child {
  padding: 0.9375rem;
  background-color: #f8e8f8;
  border-radius: 0.375rem;
  margin-top: 0.625rem;
}

/* Highlighted elements */
p.special.highlighted {
  background-color: #fff3cd;
  padding: 0.9375rem;
  border-radius: 0.375rem;
  border-left: 0.25rem solid #ffc107;
}

/* Demo links */
a:not(.back-btn) {
  color: #3498db;
  text-decoration: none;
  transition: color 0.3s ease;
}

a:not(.back-btn):hover {
  color: #2c3e50;
  text-decoration: underline;
}

/* Theory section styles */
.theory {
  background-color: #f0f7ff;
  padding: 0.9375rem;
  padding-left: 2rem;
  margin-top: 0.625rem;
  border-radius: 0.375rem;
  border-left: 0.25rem solid #2196f3;
}

.specificity {
  font-weight: bold;
  color: #e91e63;
}

.combinator-demo .parent {
  border: 0.125rem solid #333;
  padding: 0.625rem;
  margin: 0.625rem 0;
}
.combinator-demo .child {
  background: #f0f0f0;
  padding: 0.3125rem;
  margin: 0.3125rem;
}
.combinator-demo .sibling {
  border: 0.0625rem dashed #666;
  padding: 0.3125rem;
  margin: 0.3125rem;
}
.combinator-demo .highlight {
  background: #ffeb3b;
}