.body, html {
  margin: 0; padding: 0
}
/* Logo trong header */
#site-logo {
  height: 100px;
  margin-right: 0.5rem;
  vertical-align: middle;
}

/* Footer */
.site-footer {
  text-align: center;
  padding: 0.5rem 0;
  background: #f7f7f7;
  color: #555;
  font-size: 0.9rem;
  position: fixed;
  bottom: 0;
  left: 0;
  width: 100%;
  box-shadow: 0 -1px 4px rgba(0,0,0,0.1);
}

.site-footer a {
  color: #3498db;
  text-decoration: none;
}

.site-footer a:hover {
  text-decoration: underline;
}

body {
  font-family: 'Segoe UI', Tahoma, sans-serif;
  background: #eef2f5;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  min-height: 100vh;
  padding: 2rem 0;
}
.container {
  background: #fff;
  padding: 2rem;
  border-radius: 12px;
  box-shadow: 0 4px 12px rgba(0,0,0,0.1);
  width: 460px;
  text-align: center;
}
.header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 1rem;
}
.logout {
  background: #e67e22;
  color: #fff;
}
.form-group {
  margin-bottom: 1.5rem;
}
input, select {
  width: 100%;
  padding: 0.75rem;
  margin: 0.5rem 0;
  border: 1px solid #ccc;
  border-radius: 6px;
  font-size: 1rem;
}
.msg.error {
  color: #e74c3c;
  margin-top: 0.5rem;
}
.btn {
  padding: 0.75rem 1.5rem;
  margin: 0.5rem;
  border: none;
  border-radius: 6px;
  font-size: 1rem;
  cursor: pointer;
  box-shadow: 0 2px 6px rgba(0,0,0,0.1);
}
.btn.primary { background: #3498db; color: #fff; }
.btn.secondary { background: #7f8c8d; color: #fff; }
.btn.success { background: #27ae60; color: #fff; }
.btn.danger  { background: #c0392b; color: #fff; }
.btn.warning { background: #f39c12; color: #fff; }
#btn-review-due { background: #3498db; }
#btn-review-all { background: #27ae60; color: #fff; }

.link { color: #3498db; text-decoration: none; display: inline-block; margin-top: 1rem; }
.link:hover { text-decoration: underline; }

.progress-wrapper {
  margin-bottom: 1rem;
  display: flex;
  align-items: center;
  justify-content: center;
}
progress {
  width: 70%;
  height: 1rem;
  margin-right: 0.5rem;
  appearance: none;
}
progress::-webkit-progress-bar { background-color: #eee; border-radius: 6px; }
progress::-webkit-progress-value { background-color: #3498db; border-radius: 6px; }

.card {
  perspective: 1000px;
  margin-bottom: 1rem;
}
.card-inner {
  width: 100%;
  height: 240px;
  position: relative;
  transform-style: preserve-3d;
  transition: transform 0.6s;
}
.card.flipped .card-inner { transform: rotateY(180deg); }
.card-face {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
  border: 2px solid #3498db;
  border-radius: 12px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-size: 1.2rem;
  color: #2c3e50;
  padding: 1rem;
  box-sizing: border-box;
  background: #fff;
}
.back { transform: rotateY(180deg); text-align: left; align-items: start; }
.back-field { margin-top: 0.5rem; font-size: 0.95rem; }
.hidden { display: none; }