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This script uses Window.matchMedia method and the CSS prefers-color-scheme query to detect if a user has light or dark mode set as their default. If dark mode is preferred, it automatically adds the data-bs-theme="dark" attribute to the tag to enable Bootstrap's dark theme.
Use it by just including bootstrap-auto-dark-mode.js in your header.
<script src="./src/bootstrap-auto-dark-mode.js"></script>