style.css

@import url("https://fonts.googleapis.com/css?family=Montserrat:700|Montserrat+Alternates");

body * {
  text-align: center;
  font-family: "Montserrat Alternates", Arial;
}

body {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
  -ms-flex-direction: column;
  flex-direction: column;
  -webkit-box-pack: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-box-align: center;
  -ms-flex-align: center;
  align-items: center;
  background: #ccc;
}

h1 {
  font-family: Montserrat, Arial;
}

.select-screen {
  width: 60vw;
  padding: 40px 0;
  background-color: #999;
  border: 3px solid #5f5f5f;
  border-radius: 3px;
}

.select-screen button {
  width: 15vw;
  padding: 15px;
  margin: 0 20px;
  font-size: 1.3em;
  background: #b9b9b9;
  border: none;
  border-radius: 2px;
}

.select-screen button:hover {
  background: #c9c9c9;
}

.select-screen button:focus {
  outline: 0;
}

#select-xo-screen {
  display: none;
}

#game-screen {
  display: none;
}

#game-field {
  height: 300px;
  width: 320px;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: justify;
  -ms-flex-pack: justify;
  justify-content: space-between;
  -webkit-box-align: space-between;
  -ms-flex-align: space-between;
  align-items: space-between;
  -ms-flex-wrap: wrap;
  flex-wrap: wrap;
}

#game-field .square {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-align: center;
  -ms-flex-align: center;
  align-items: center;
  -webkit-box-pack: center;
  -ms-flex-pack: center;
  justify-content: center;
  width: 33%;
  height: 33%;
  background: #eee;
  border-radius: 3%;
  font-size: 3em;
}

#game-field .square:hover {
  background: #f9f9f9;
}

#restart-btn {
  display: none;
  margin-top: 15px;
  padding: 10px 30px;
  border: none;
  border-radius: 2px;
  font-size: 1.3em;
  background: #d9d9d9;
}

#restart-btn:hover {
  background: #e1e1e1;
}

footer {
  position: fixed;
  bottom: 0;
  width: 100%;
  margin: 0;
  padding: 5px;
  opacity: 0.8;
}