:root {
  --border: 8px;
  --cell-border: 2px;
  --cell-font-size: 2em;
  /* Light theme (default) */
  --bg-color: #ffffff;
  --text-color: #000000;
  --border-color: #000000;
  --input-bg: transparent;
  --user-input-bg: #lightgray;
  --focus-bg: burlywood;
  --solved-color: #555;
}

[data-theme="dark"] {
  /* Dark theme */
  --bg-color: #1a1a1a;
  --text-color: #ffffff;
  --border-color: #ffffff;
  --input-bg: #2d2d2d;
  --user-input-bg: #3d3d3d;
  --focus-bg: #4a4a3d;
  --solved-color: #aaa;
}

h1 {
  text-align: center;
  font-family: Verdana, Geneva, Tahoma, sans-serif;
}

body {
  background-color: var(--bg-color);
  color: var(--text-color);
  transition: all 0.3s ease;
}

table {
  border-collapse: collapse;
  margin: 20px auto;
}

td {
  border: var(--cell-border) solid var(--border-color);
  width: 55px;
  height: 55px;
  text-align: center;
  position: relative;
  padding: 0;
}

/* Add thicker borders for the 3x3 boxes */
td:nth-child(3n) {
  border-right: var(--border) solid var(--border-color);
}

td:nth-child(3n + 1) {
  border-left: var(--border) solid var(--border-color);
}

tr:nth-child(3n) td {
  border-bottom: var(--border) solid var(--border-color);
}

tr:nth-child(3n + 1) td {
  border-top: var(--border) solid var(--border-color);
}

.cell:focus {
  outline: none;
  background-color: var(--focus-bg);
  caret-color: transparent;
}

input {
  width: 100%;
  height: 100%;
  text-align: center;
  border: none;
  box-sizing: border-box;
  background-color: var(--input-bg);
  color: var(--text-color);
  padding: 0;
  margin: 0;
}

.user-input {
  font-weight: bold;
  font-size: var(--cell-font-size);
  background-color: var(--user-input-bg);
}

.solved-cell {
  font-weight: bolder;
  font-size: var(--cell-font-size);
  color: var(--solved-color);
}

.button-container {
  display: flex;
  justify-content: space-between;
  margin: 20px auto;
  width: 100%;
  max-width: 500px;
}

.button-container button {
  flex: 1;
  margin: 0 10px;
  font-family: Verdana, Geneva, Tahoma, sans-serif;
  font-size: 16px;
  padding: 10px;
  cursor: pointer;
  color: white;
  border: none;
  border-radius: 5px;
  transition: background-color 0.3s;
}

#solve-button {
  background-color: green;
}

#solve-button:hover {
  background-color: lightgreen;
}

#reset-button {
  background-color: red;
}

#reset-button:hover {
  background-color: lightcoral;
}

footer {
  text-align: center;
  margin-top: 20px;
  font-family: Verdana, Geneva, Tahoma, sans-serif;
}

/* From Uiverse.io by cuzpq */
.theme-checkbox {
  position: relative;
  top: unset;
  right: unset;
  margin: 0 auto 20px;
  display: block;
  --toggle-size: 12px;
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  width: 4em;
  height: 2em;
  background: linear-gradient(to right, #efefef 50%, #2a2a2a 50%) no-repeat;
  background-size: 205%;
  background-position: 0;
  transition: 0.4s;
  border-radius: 99em;
  cursor: pointer;
  font-size: var(--toggle-size);
}
.theme-checkbox::before {
  content: "";
  width: 1.5em;
  height: 1.5em;
  position: absolute;
  top: 0.25em;
  left: 0.25em;
  background: linear-gradient(to right, #efefef 50%, #2a2a2a 50%) no-repeat;
  background-size: 205%;
  background-position: 100%;
  border-radius: 50%;
  transition: 0.4s;
}
.theme-checkbox:checked::before {
  left: calc(100% - 1.5em - 0.25em);
  background-position: 0;
}
.theme-checkbox:checked {
  background-position: 100%;
}