Go to Website, Site Designs, and Select any of the Websites to enter the Site Builder.
Clicking the Add Block will Show a Side Menu that has the different Items to Add. Drag and Drop the Spacer from the Elements.
Select the Spacer and Add it between Sections. Give it a different Color according to your preference. Click the Spacer and for more Settings Follow the Image below.
In the Dimensions, you can Increase and Decrease the Width and Height.