* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html,
body,
button {
  height: 100%;
  background-color: #81E979;
  font-family: "Source Sans Pro", sans-serif;
}

.container {
  max-width: 600px;
  margin: 0 auto;
  padding: 40px 15px;
  text-align: center;
  color: #ffffff;
}

#calculation {
  padding-bottom: 10px;
  font-size: 3em;
  font-weight: bolder;
  text-transform: uppercase;
}

#stats {
  padding-bottom: 40px;
  font-size: 24px;
}

.score {
  margin-bottom: 10px;
  font-size: 12px;
}

.score span {
  font-size: 24px;
}

.levels button {
  background-color: #9CCC65;
  color: white;
  font-size: 18px;
  border: none;
  padding: 10px 25px;
  display: inline-block;
  cursor: pointer;
  margin: -4px;
  transition: all 0.2s ease;
  outline: none;
}

.levels button.selected {
  background-color: #B0FE76;
}

.levels button:hover {
  background-color: #b0fe76;
}

.levels button:disabled,
.levels button:disabled:hover {
  cursor: not-allowed;
  ;
  background-color: #9CCC65;
}

#solution-container {
  width: 70%;
  margin: 0 auto 20px;
}

.solution {
  border: 3px solid #b0fe76;
  background-color: #81e979;
  color: #ffffff;
  display: inline-block;
  width: 25%;
  padding-bottom: 25%;
  margin: -3px;
  position: relative;
  cursor: pointer;
  transition: all 0.2s ease-in-out;
}

.solution.fixed.correct,
.solution.fixed.correct:hover {
  background-color: #42A5F5;
}

.solution.fixed.correct .number {
  transform: translate(-50%, -50%) scale(2);
}

.solution.fixed .number {
  transform: translate(-50%, -50%) scale(0.8);
}

.solution.fixed.incorrect,
.solution.fixed.incorrect:hover {
  background-color: #F44336;
}

.solution.fixed:hover {
  background-color: #81e979;
}

.solution:hover {
  background-color: #b0fe76;
}

.number {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  font-size: 1.5em;
  transition: all 0.2s ease-in-out;
}

.next button {
  background-color: #ffffff;
  color: #81e979;
  font-size: 18px;
  border: none;
  padding: 10px 25px;
  margin: 0 20px;
  display: inline-block;
  cursor: pointer;
  outline: none;
  transition: all 0.2s ease;
}

#reset {
  background-color: #2196F3;
  color: white;
}

.next button:hover {
  box-shadow: 0 2px 2px rgba(0, 0, 0, 0.2);
}