Start styling output box. Remove readonly.

public/style.css

@@ -0,0 +1,16 @@
+#output-area {
+    border-right: 2px solid grey;
+    width: 30ch;
+}
+
+#output-area:focus {
+    outline: none;
+}
+
+#gutter {
+    background-color: rgb(230, 230, 230);
+}
+
+#output {
+    display: flex;
+}