First let me simplify. A theme is simply a FRAME for you Picture... Which is your Content. WordPress has no professional standards so every theme is different. But the FRAME should not impact your Painting (Your Content)

How to change themes in WordPress 4.4?

A theme gives your live site a different look and feel that includes page layout, widgets, accent colours, and fonts. WordPress have its theme library from where you can download or search for a specific theme and load it onto your website. There are also third party websites like themeforest.com, from where you can also download your desired themes. The installation steps change when the source changes from WordPress.com. You need to test the themes for conflicts with the pre-existing widgets and tools on your website. We'll discuss how to change the theme on your WordPress 4.4 website via the WordPress site dashboard.