site.css

body,
html {
  padding: 0;
  margin: 0;
}

button {
  border: 0;
  background-color: orangered;
  color: white;
  font-size: 1em;
  padding: 0.5em 1em;
  text-transform: uppercase;
}

button[disabled] {
  background-color: lightgray;
  color: grey;
  cursor: not-allowed;
}

button:active {
  background-color: darkred;
}

input {
  border: 0;
  border-bottom: 2px solid #ccc;
  box-sizing: border-box;
  font-size: 1em;
  margin-bottom: 1em;
  outline: none;
  padding: 0.5em;
}

input:focus {
  border-bottom: 2px solid orangered;
}

main {
  display: flex;
  flex-direction: row;
  flex-wrap: nowrap;
  justify-content: center;
}

#board-holder {
  flex: 0 0 525px;
}

#board-squares {
  display: grid;
  grid-template-columns: repeat(7, 75px);
  grid-template-rows: repeat(6, 75px);
}

#click-targets {
  display: grid;
  grid-template-columns: repeat(7, 1fr);
}

#form-holder {
  display: flex;
  flex: 0 0 150px;
  flex-direction: column;
  padding: 1em;
}

#overlay {
  background-image: url(./images/connect-four.png);
  background-size: cover;
  height: 450px;
  position: absolute;
  top: 75px;
  width: 525px;
}

.click-target {
  background-color: transparent;
  box-sizing: border-box;
  border: 1px solid #ccc;
  border-radius: 65px;
  height: 65px;
  margin: 5px;
  width: 65px;
  transition: background-color ease-in-out 0.25s;
}

.is-invisible {
  visibility: hidden;
}

.token {
  box-sizing: border-box;
  border-radius: 65px;
  height: 65px;
  margin: 5px;
  width: 65px;
}

.token.red {
  background-color: red;
}

.token.black {
  background-color: black;
}

.red .click-target:hover {
  background-color: red;
}

.black .click-target:hover {
  background-color: black;
}

.full.click-target {
  border: 1px solid white;
}

.full.click-target:hover {
  background-color: white;
  cursor: no-drop;
}