Every now and then I check out the new options in the available themes for a WordPress blog. I am one of those people who likes to change things every now and then. I prefer to see how my blog with my pages, posts, etc., will appear with the new theme. I realized that under the new WordPress Admin menu when you select a theme it only allows you to see a generic blog. However; under the older Admin menu you can see how your current blog will look with the new theme before it is activated.
From the side menu click on WP Admin.
If your blog does not have that option at the bottom of the screen add: /wp-admin to your site address. For example: https://Yourblognamehere.wordpress.com/wp-admin The page will then look like this:
Then click on Appearance; and Themes. When your mouse hovers over a theme there will be an option to preview the theme:
Click on the Preview button. So for the one above this is how my blog would appear if I selected it without any customizations:
When I go through and make changes to the options available I can see how they will look on my blog. Only when I am ready to activate and save the new theme will it go live on the blog.