Get CSS of selected component ?

You could listen the event component:selected, then you could get the component's DOM element from that. Afterwards, you could simply use javascript to get style of that component. About getting style by javascript, you can check this link How to get CSS values in JavaScript

Here is an example:

editor.on('component:selected', (component) => {
  const domElement = component.getEl();
  const style = window.getComputedStyle(domElement)
})