*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  font-size: 10px;
}

html,
body {
  margin: 0;
  padding: 0;
  height: 100vh;
  width: 100vw;
}

body {
  background-color: #eee;
  display: flex;
  flex-direction: column;
  justify-content: space-evenly;
  align-items: center;
  font-size: 1.6rem;
  text-align: center;
  font-family: 'Potta One', cursive;
}

header * {
  margin: 0;
}

main {
  background-color: #151515;
  height: 90vw;
  width: 90vw;
  max-height: 700px;
  max-width: 700px;
  border-radius: 2.5rem;
  display: flex;
  flex-wrap: wrap;
  align-content: center;
  box-shadow: 0 0 20px black;
  color: white;
  font-size: 6rem;
}

.cell {
  width: 33%;
  height: 33%;
  transition: 500ms;
  display: flex;
  justify-content: center;
  align-items: center;
  text-align: center;
}

.cell-empty:hover {
  background-color: #202020;
  cursor: pointer;
}

.cell:nth-of-type(2),
.cell:nth-of-type(5),
.cell:nth-of-type(8) {
  border-left: 2px solid white;
  border-right: 2px solid white;
}

.cell:nth-of-type(4),
.cell:nth-of-type(5),
.cell:nth-of-type(6) {
  border-top: 2px solid white;
  border-bottom: 2px solid white;
}

.cell:nth-of-type(1) {
  border-radius: 2.5rem 0 0 0;
}

.cell:nth-of-type(3) {
  border-radius: 0 2.5rem 0 0;
}

.cell:nth-of-type(7) {
  border-radius: 0 0 0 2.5rem;
}

.cell:nth-of-type(9) {
  border-radius: 0 0 2.5rem 0;
}

.end-message {
  z-index: 10;
  display: flex;
  position: fixed;
  width: 25rem;
  height: 20rem;
  top: 50%;
  left: 50%;
  margin-bottom: -10rem;
  margin-left: -12.5rem;
  flex-direction: column;
  justify-content: space-evenly;
  align-items: center;
  background-color: #151515;
  color: #eee;
  font-size: 2rem;
  padding: 2rem;
  border-radius: 2rem;
  box-shadow: 0 0 20px #eeeeee33;
  transition: 500ms;
}

.end-message > * {
  margin: 0.5rem;
}

button {
  appearance: none;
  -moz-appearance: none;
  -webkit-appearance: none;
  border: none;
  cursor: pointer;
  font-size: 1.75rem;
  padding: 0.75rem 1.5rem;
  font-family: inherit;
  border-radius: 2.5rem;
  display: flex;
  justify-content: center;
  align-items: center;
  text-align: center;
}

.reset-btn {
  background-color: #eee;
  color: #151515;
  box-shadow: 0 0 20px #eeeeee33;
}

#instructions {
  text-align: left;
  display: flex;
  flex-direction: column;
  justify-content: space-evenly;
  align-items: center;
  padding: 1rem;
}

.start-btn {
  background-color: #151515;
  color: #eee;
}

@media (min-width: 426px) {
  html {
    font-size: 15px;
  }
}

@media (min-width: 769px) {
  .end-message {
    opacity: 0.25;
  }
  .end-message:hover {
    opacity: 1;
  }
}

@media (min-width: 1025px) {
  html {
    font-size: 20px;
  }
}