There's no need to preview your website. The Website Editor shows you exactly how your website is going to look to your visitors when you edit blocks or add your own content. The website templates have been designed to automatically update when you make changes, so there's no need for you to adjust spacing or margins. When you move blocks, they will fit precisely where you want them to fit, same with the elements inside the block.
Now you can focus on adding your content and rest assured that your website visitors will see exactly what you see when you edit your site.