Firefox by default uses a default theme that take colors from the color accent currently set on your personalization options on Windows 10. However, the web browser from Mozilla includes an additional light and dark themes that brings the same Microsoft Edge look and feel.
If you want to switch the current Firefox theme, it’s a pretty easy task. You just need to know where to look.
How to change themes on Firefox
Right-click the main menu button on the top-right.
Select the Customize option.
At the bottom of the page, click the Themes drop-down menu, and select Light or Dark depending on the theme you want to use.
Click the Done button.
Alongside the light and dark theme, remember that you can also apply other custom-designed themes.