shoelace/docs/getting-started/customizing.md

Customizing

Shoelace components can be customized at a high level through design tokens. This gives you control over theme colors and general styling. For more advanced customizations, you can make use of component parts and custom properties to target individual components.

Design Tokens

Shoelace makes use of several design tokens to provide a consistent appearance across components. You can customize them and use them in your own application with pure CSS — no preprocessor required.

Design tokens offer a high-level way to customize the library with minimal effort. There are no component-specific variables, however, as design tokens are intended to be generic and highly reusable. To customize an individual component, refer to the section entitled Component parts.

Design tokens are CSS custom properties ("CSS variables") that are defined in the :root block of shoelace.css. This stylesheet is imported when you install Shoelace, so design tokens are available on your page at that point. Because design tokens are global, they're always prefixed with --sl to avoid collisions with other libraries.

To customize a design token, simply override it in your stylesheet using a :root block.

:root {
  /* Changes the primary color to a shade of orange at 90% saturation */
  --sl-color-primary-hue: 30;
  --sl-color-primary-saturation: 90%;
}

Many design tokens are described further along in this documentation. For a complete list, refer to shoelace.scss in the project's source code.

!> Never modify variables directly in shoelace.css because your changes will be overwritten when you upgrade the library. Even if you don't plan on upgrading, it's always better to override design tokens in your own stylesheet for better maintainability.

Component Parts

Whereas design tokens offer a high-level way to customize the library, component parts offer a low-level way to customize individual components. Again, this is done with pure CSS — no preprocessor required.

Shoelace components use a shadow DOM to encapsulate their styles and behaviors. As a result, you can't simply target their internals with the usual CSS selectors. Instead, components expose "parts" that can be targeted with the CSS part selector, or ::part().

Here's an example that modifies buttons with the tomato-button class.

<sl-button class="tomato-button">
  Tomato Button
</sl-button>

<style>
  .tomato-button::part(base) {
    background: white;
    border: solid 1px tomato;
  }

  .tomato-button::part(base):hover {
    background: rgba(255, 99, 71, .1);
  }  

  .tomato-button::part(base):active {
    background: rgba(255, 99, 71, .2);
  }    

  .tomato-button::part(base):focus {
    box-shadow: 0 0 0 3px rgba(255, 99, 71, .33);
  }

  .tomato-button::part(label) {
    color: tomato;
  }
</style>

At first glance, this approach might seem a bit verbose or even limiting, but it comes with a few important advantages:

• Customizations can be made to components with explicit selectors, such as ::part(icon), rather than implicit selectors, such as .button > div > span + .icon, that are much more fragile.

• The internal structure of a component will likely change as it evolves. By exposing component parts through an API, the internals can be reworked without fear of breaking customizations as long as its parts remain intact.

• It encourages us to think more about how components are designed and how customizations should be allowed before users can take advantage of them. Once we opt a part into the component's API, it's guaranteed to be supported and can't be removed until a major version of the library is released.

Most (but not all) components expose parts. You can find them in each component's API documention under the "CSS Parts" section.

Custom Properties

For convenience, some components expose CSS custom properties you can override. These are not design tokens, nor do they have the same --sl- prefix since they're scoped to a component.

You can set custom properties on a component in your stylesheet.

sl-avatar {
  --size: 6rem;
}

This will also work if you need to target a subset of components with a specific class.

sl-avatar.your-class {
  --size: 6rem;
}

Alternatively, you can set them inline directly on the element.

<sl-avatar style="--size: 6rem;"></sl-avatar>

Not all components expose CSS custom properties. For those that do, they can be found in the component's API documentation.