Each element has a list of Element Properties. They are not the same for each element; each element only includes the properties that apply to it.
Once you have selected an element, its properties will appear in the Properties Pane at the righthand side of the Designer window just below the Element Description. To change any property of an element, first uncheck Use Theme Value beside the property you want to change. Adjust the default values for the property to whatever you desire and click Save. See the sections on each Element Property for its options and specific details on how to change its values.