Hey, Godstime Onyibe! Currently, the code is stable and works fine.
Add the script before the closing tag body, as it follows:
...
<script src="cdn.jsdelivr.net/gh/luizfilipezs/dark-theme-switc…"></script>
<script type="text/javascript">
new ThemeSwitcherGenerator().init();
new ThemeSwitcher(someElement).init();
</script>
</body>
Remember that what the package does is inverting the colors. So, to change the background of your body tag, as an example, you must give it a background color.