@pantarmj I’m still using Ionic 3 in my main app (it’s already in production, so I’m not thinking in migrating to Ionic 4 yet) and have not used Ionic 4 very much. I don’t know if there is a way to style a component specific to a platform without doing something in the ts file (like adding a dynamic class to the component, based on the platform).
Assuming there is already a class in the html of your custom element that specifies the platform (let’s say, <my-component class="ios ..." ...> ... </my-component>), then you should be able to access with :root.ios { ... }, where root refers to your component (assuming it’s the css of your custom component itself), or with my-custom-component.ios { ... } to access from the outside of the component.