At present, global Sass definitions (variables, mixins, functions) need to be added in the full code view: Settings > View > Full code editor.
Once you’ve moved your mixin definitions to the full code editor, you can @include them anywhere in the regular view.
The code editor in the main view is only intended for single selectors on the whole. There is a bit of wiggle room with that, but when it comes to Sass Microthemer’s compilation optimisation process does not allow definitions in the editor designed for single selectors. In an upcoming update, you will be able to add “Sass snippets” to folders, which will allow you to enter any kind of Sass and keep things more organised.
