body {
  display: flex;
  flex-direction: column;
  align-items: center;
}

#timer {
  margin-bottom: 10px;
}

.controls {
  display: flex;
  gap: 10px;
  align-items: center;
  margin-bottom: 10px;
}

.app {
  display: none;
  grid-template-columns: repeat(9, 1fr);
  gap: 1px;
  background-color: #000000;
  width: fit-content;
  border: 3px solid #000000;
}

.app.visible {
  display: grid;
}

.app div {
  width: 60px;
  height: 60px;
  background-color: #ffffff;
  border: 1px solid #cccccc;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 20px;
}

.app input {
  width: 100%;
  height: 100%;
  border: none;
  text-align: center;
  font-size: 20px;
}

.app .correct {
  color: #009900;
}

.app .incorrect {
  color: #ff0000;
}

.app .grid-header,
.app .grid-operator-header,
.app .grid-column-header,
.app .grid-row-header {
  background-color: #e0e0e0;
}