You’ll find a wide variety of launch themes in the Add-ons site to help you show off your current mood or interest. We’ve even added support for vertical tabs, a new feature you Insiders have been helping us test. Your tabs, address bar, and other parts of the browser will also take on the look of the theme for a more immersive experience. When you apply a theme, you’ll notice a new background on your new tab page.
They can also be helpful for distinguishing your different profiles to help you separate home, school or work accounts.
Themes change the look and feel of your browser to create a beautiful and immersive visual experience.