:root {
  --cell-size: 100px;
  --mark-size: 90px;
}

* {
  box-sizing: border-box;
}

body {
  height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: whitesmoke;
  flex-direction: column;
}

.main-box {
  display: grid;
  grid-template-columns: repeat(3, 100px);
}

.inner-box {
  height: var(--cell-size);
  width: var(--cell-size);
  border: 1px solid black;
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
  cursor: pointer;
}

.inner-box.x,
.inner-box.circle {
  cursor: not-allowed;
}

.inner-box:nth-child(1),
.inner-box:nth-child(2),
.inner-box:nth-child(3) {
  border-top: none;
}
.inner-box:nth-child(3n + 1) {
  border-left: none;
}
.inner-box:nth-child(3n + 0) {
  border-right: none;
}
.inner-box:nth-child(7),
.inner-box:nth-child(8),
.inner-box:nth-child(9) {
  border-bottom: none;
}

/* X mark */
.inner-box.x::before,
.inner-box.x::after {
  content: "";
  position: absolute;
  width: calc(var(--mark-size) * 0.15);
  height: var(--mark-size);
  background-color: black;
}
.inner-box.x::before {
  transform: rotate(45deg);
}
.inner-box.x::after {
  transform: rotate(-45deg);
}

/* O mark */
.inner-box.circle::before,
.inner-box.circle::after {
  content: "";
  position: absolute;
  border-radius: 50%;
}
.inner-box.circle::before {
  width: var(--mark-size);
  height: var(--mark-size);
  background-color: black;
}
.inner-box.circle::after {
  width: calc(var(--mark-size) * 0.7);
  height: calc(var(--mark-size) * 0.7);
  background-color: white;
}

.winning-message {
  font-size: 5rem;
  display: flex;
  align-items: center;
  justify-content: center;
  margin-top: 5vh;
}

.restart {
  background-color: black;
  padding: 10px;
  color: white;
  font-size: 2rem;
  cursor: pointer;
}