.flash-card-holder {
  display: flex;
  flex-wrap: wrap;          /* allow cards to wrap to next line */
  gap: 1rem;                /* spacing between cards */
  justify-content: center;  /* center cards horizontally */
  align-items: stretch;     /* make cards equal height if possible */
  padding: 1rem;
}

.flash-card-holder .card {
  display: flex;
  flex: 1 1 250px;          /* grow/shrink, minimum width 250px */
  flex-direction: column;
  max-width: 300px;         /* prevent cards from getting too wide */
  justify-content: center;
  padding: 10px;
  text-align: center;
  margin: 10px;
}

.card-tall {
  min-height: 300px;
}