https://www.theverge.com/2018/6/5/17424580/mozilla-firefox-experimental-extensions-color-side-view

Mozilla Firefox is testing out two new experimental extensions that let users further customize their browser and view tabs together more easily. The first one is called Firefox Color and it lets you change the colors of the background, text, icons, and the toolbar. Whatever you change can be saved and you can also choose to share your color schemes with others.

Firefox also has a selection of pre-made color themes that you can choose from if you don’t want to customize every single color yourself.