When the user's browser is running »dark mode«, the dark color scheme will be used automatically. The default is the light/white color scheme. Check out the [`style.html`](https://github.com/janraasch/hugo-bearblog/blob/master/layouts/partials/style.html)-file for the implementation.
Please check out the [hugo.toml](https://github.com/janraasch/hugo-bearblog/blob/master/exampleSite/hugo.toml) included in the [exampleSite](https://github.com/janraasch/hugo-bearblog/tree/master/exampleSite) of this theme.
If you are starting fresh, simply copy over the contents of the `exampleSite`-directory included in this theme to your source directory. That should give you a good idea about how things work, and then you can go on from there to make the site your own.
Add a `custom_head.html`-file to your `layouts/partials`-directory. In there you may add a `<style>`-tag, *or* you may add a `<link>`-tag referencing your own `custom.css` (in case you prefer to have a separate `.css`-file). Check out the [`style.html`](https://github.com/janraasch/hugo-bearblog/blob/master/layouts/partials/style.html)-file to find out which CSS-styles are applied by default.