V1

style.css

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+body {
+    /* Background pattern from Toptal Subtle Patterns */
+    background: url("handmadepaper.png");
+    text-align: center;
+}
+
+h1 {
+    text-align: center;
+    text-shadow: 1px 1px grey;
+    text-decoration: underline;
+}
+
+table {
+    border-spacing: 0;
+    border-collapse: collapse;
+}
+
+div {
+    display: flex;
+    column-gap: 0.5em;
+    row-gap: 0.5em;
+    margin: 1em auto;
+    justify-content: center;
+}
+
+input[type="radio"] {
+    margin-top: -1px;
+    vertical-align: middle;
+}
+
+label.disabled {
+    color: #aaa;
+}
+
+.grid td {
+    border: 2px solid black;
+    padding: 0;
+}
+
+.region td {
+    border: 1px solid grey;
+}
+
+.grid input {
+    width: 1.6em;
+    height: 1.6em;
+    font-size: 1.5em;
+    border: 0;
+    padding: 0;
+    text-align: center;
+}
+
+.grid input:enabled {
+    font-family: cursive;
+    color: darkblue;
+}
+
+.grid input:disabled {
+    color: black;
+    font-weight: bold;
+}
+
+.grid input::placeholder {
+    color: #888;
+}
+
+.unhighlighted:enabled {
+    background: transparent;
+}
+
+.highlighted {
+    background: yellow;
+}
+
+#shareDiv {
+    display: block;
+    line-height: 80%;
+}
+
+#shareA {
+    text-decoration: none;
+    font-size: 0.8em;
+    letter-spacing: 0.5em;
+}
+
+#highlightRadiosDiv {
+    column-gap: 0;
+}