In case this can help anyone, here’s simple CSS as a jumping off point. Turns it into a basic rounded square big enough for thumbs with a toggled background color.
input[type="checkbox"] {
-webkit-appearance:none;
width:30px;
height:30px;
background:white;
border-radius:5px;
border:2px solid #555;
}
input[type="checkbox"]:checked {
background: #abd;
}<input type="checkbox" />