<script src="path/to/themewidget.js" charset="utf-8"></script>
That's all the code it now takes for me to add a theme switcher widget on any of my projects using my custom SASS Theme Frameworks.
You can demo it on my personal page which is a work in progress. The widget is on the bottom-right edge of the page.
It supports a couple of different theme options which I will later add more of. It also has 4 different theme varients to choose from which effect the dark or light theme type.
It works by fetching SCSS resources and SCSS.js to compile to a CSS file. It is far from optimized but it works with only a couple of seconds of delay to update the theme. It also uses the local storage API to store the theme and update it on page load. I eventually want to store the theme across every domain using the widget so it will be like a universal theme.