_module-x.scss

.module-x {
    background-color: red;
}

/**
 * Exposed API
 */

@mixin module-x-blue {
    background-color: blue;
}

layout-x.scss

.layout-x {}

.layout-x .module-x {

    /**
     * @smallscreen
     */

    @media all and (max-width: 400px) {

        // Call the module-x-blue 'api'
        @include module-x-blue;
    }
}