.theory-card {
  perspective: 1000px;
  margin: 8px auto;
}
.theory-card__inner {
  position: relative;
  width: 100%;
  height: 100%;
  transform-style: preserve-3d;
  transition: transform .6s;
  cursor: pointer;
}
.theory-card__inner.is-flipped {
  transform: rotateY(180deg);
}
.theory-card__face {
  position: absolute;
  inset: 0;
  border-radius: 12px;
  backface-visibility: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
}
.theory-card__front {
  background: #f0f9ff;
  border: 2px solid #3b82f6;
}
.theory-card__back {
  background: #ecfdf5;
  border: 2px solid #10b981;
  transform: rotateY(180deg);
}
.theory-card__front-text {
  font-size: 1.2em;
  font-weight: 700;
  text-align: center;
  padding: 12px;
  line-height: 1.25;
}
.theory-card__back-content {
  font-size: 1.05em;
  text-align: center;
  padding: 16px;
  overflow: auto;
}
.theory-card__front-text,
.theory-card__back-content {
  -webkit-user-select: none;
  user-select: none;
}