WordPress allows you to install thousands of themes which are available from there theme directory, theme change the layout and appearance of your blog/website themes can be customised to your own branding etc.
These steps below will guide you through how you can install themes on your WordPress hosting account
Login to the WordPress dashboard for your site. (yourdomain.com/wp-admin)
Select Appearance and Themes from the menu
Click Add New
Using the search box or categories, search for the theme you wish to install
Your theme will now be installed
The appearance screen will now load and you will see all your installed themes which you can customise