# Edit a Mixin

Theme mixins are partial theme files that are always applied on top of a base theme. The concept is very similar to [JavaScript mixins](https://javascript.info/mixins).

You can edit a theme mixin in two ways:

* **Open a theme mixin file**

  Click the ![load Theme.Image](https://4196911125-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FyrHxzbanglDTTaYfB34Z%2Fuploads%2Fgit-blob-48a4925336fce1d57c28997bf3827d01812a6ee6%2FOpenFolder.png?alt=media) Open Theme button on the toolbar and select the theme mixin file (.mixin.theme) to load.
* **Double click a theme mixin file**

  You can double click .mixin.theme file in a folder or in a **Visual Studio** project to load it in Theme Builder.

{% hint style="info" %}
You cannot create a theme mixin using the Theme Builder. Theme mixins are created in Visual Studio by adding the **Theme Mixin** component to the project, or manually by creating a new .mixin.theme file.
{% endhint %}

## Switch the Base Theme

While editing a theme mixin you can switch the base theme by selecting one of the preloaded themes, or by again clicking the ![load Theme.Image](https://4196911125-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FyrHxzbanglDTTaYfB34Z%2Fuploads%2Fgit-blob-48a4925336fce1d57c28997bf3827d01812a6ee6%2FOpenFolder.png?alt=media) Open Theme button and selecting a .theme file.

When you change the base theme, the Theme Builder updates the previews but the tree and the editor are still limited to the elements defined in the mixin.

## See Also