Add rollback button when only one theme is installed

[DevinWalker]

Description

When only one theme is installed in a WordPress site the theme page changes from a card layout to a single theme detail view. We never tested this and thus never have supported it.

This was uncovered here: https://wordpress.org/support/topic/using-wp-rollback-on-elementor-hello-theme/

How Has This Been Tested?

• Single theme installed
• Multiple themes installed

Screenshots (jpeg or gifs if applicable):

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code follows has proper inline documentation.