Theme mixins are partial theme files that are always applied on top of a base theme. The concept is very similar to JavaScript mixins.
You can edit a theme mixin in two ways:
Open a theme mixin file
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.
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.
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.
Click the Open Theme button on the toolbar and select the theme mixin file (.mixin.theme) to load.
While editing a theme mixin you can switch the base theme by selecting one of the preloaded themes, or by again clicking the Open Theme button and selecting a .theme file.