:root {
  --background: white;
  --incorrect: red;
  --correct: green;
  --link: blue;
}

body {
  background-color: var(--background);
  max-width: 1000px;
  margin: auto;
}

.incorrect {
  color: var(--incorrect);
}

.correct {
  color: var(--correct);
}

a:link {
  color: var(--link);
}

.header > .incorrect {
  color: red;
}

.header > .correct {
  color: green;
}

.header > a:link {
  color: blue;
}

.header {
  background-color: #a0a0a0;
  text-align: center;
  margin: 0 auto;
}

.header a {
  padding: 0 12px;
}

.box {
  display: inline-block;
  margin-top: 1px;
  margin-bottom: 1px;
  margin-left: -1px;
  margin-right: -1px;
  border: 1px solid black;
}

.grid {
  border-collapse: collapse;
}

.grid td {
  height: 30px;
  width: 30px;
  background-color: white;
  border: 1px solid black;
  text-align: center;
}

.grid td.unbordered {
  border: none;
}

.grid td.default-background {
  background-color: transparent;
}

.grid td.filled {
  background-color: black;
}

ul.with-header:before{
  content: attr(header-text);
  font-weight:bold;
  margin-left: -20px;
}

.hint {
  background-color: black;
}

.hint:active {
  background-color: transparent;
}