How does opacity work?
The opacity of objects can be adjusted. This is most commonly used to 'fade out' objects and put emphasis on the other objects.
When you want to reduce the opacity of the entire object you can adjust the the pass through percentage in the Layer section of the right pane.
If you only want to change the opacity of a certain part of the object. You have to select the fill or stroke and reduce the percentage next to the HEX code.
If you only see a name of a colour, you need to break the style first by clicking the "Detach style" button.