Shoelace is designed to be highly customizable through pure CSS. A simple, pleasing base theme is provided as well as an official [dark theme](#dark-mode).
The base theme must always be loaded first, even if you want to use another theme exclusively. It contains important, reusable design tokens that many components rely on.
For developers, themes are also available as JavaScript modules that export [Lit CSSResult](https://lit.dev/docs/api/styles/#CSSResult) objects. You can find them in `dist/themes/*.styles.js`.
A theme is nothing more than a stylesheet that uses the Shoelace API to customize design tokens and/or components. To create a theme, you will need a decent understanding of CSS, including [CSS Custom Properties](https://developer.mozilla.org/en-US/docs/Web/CSS/--*) and the [`::part` selector](https://developer.mozilla.org/en-US/docs/Web/CSS/::part).
The recommended way to create a theme is with a separate stylesheet that piggybacks on top of the base theme. You can [customize design tokens](/getting-started/customizing#design-tokens) and [style components](/getting-started/customizing#component-parts) to achieve the look you want. By working _with_ the base theme instead of replacing it, your theme will remain lightweight and "future proof", as upcoming versions of Shoelace may introduce new design tokens that your theme won't have.
All theme classes must use the `sl-theme-{name}` convention, where `{name}` is a lowercase, hyphen-delimited value representing the name of your theme. For example, a theme called "Purple Power" would use the `sl-theme-purple-power` class.
Every selector in a theme must be scoped to the theme's class as to ensure interoperability with other themes. You should also scope them to `:host` so they can easily be imported and applied to third-party components.
[Design tokens](/getting-started/customizing#design-tokens) give you a high-level way to customize Shoelace components. You can override them in your theme as shown below.
!> Avoid scoping design tokens to `:root`. You may notice that the base theme does this, but that's because it's not technically a theme — it's a set of design tokens and base styles that themes can use as a foundation to build upon.
To customize individual components, use the following syntax to target [component parts](/getting-started/customizing#component-parts). Available parts can be found in the "CSS Parts" section of each component's documentation.
Some components offer additional customizations through [custom properties](http://localhost:4000/#/getting-started/customizing?id=custom-properties). If available, they will be listed in the "CSS Custom Properties" section of the documentation.
The built-in dark theme uses an "inverted" color scale, so if you're using design tokens as intended, you'll get a decent dark mode for free. While this isn't the same as a professionally curated dark theme, it provides an excellent baseline for one and you're encouraged to customize it further depending on your specific needs.
To install the dark theme, add the following to the `<head>` section of your page.
Shoelace doesn't try to auto-detect the user's light/dark mode preference. This should be done at the application level. As a best practice, to provide a dark theme in your app, you should:
Shoelace avoids using the `prefers-color-scheme` media query because not all apps support dark mode, and it would break things for the ones that don't.