To use a theme of your choice, simply load its corresponding CSS file.
For webpack builds and similar, you’d do something like:
Themes in browser environments
Otherwise, if you’re constrained to a browser, just link the CSS file as a regular stylesheet.
<link rel="stylesheet" type="text/css" href="https://npmcdn.com/flatpickr/dist/themes/dark.css">
Available theme names are listed in the theme selector on top of the page.