If you open its home page for the first time, you may notice that its logo jumps for a second. The tap on the "Firefox Nightly" logo itself will change the theme of the browser between 3 different options. These options are Dark, Light and Blue where all 3 are themed with Nightly colours.
It turns out that in settings you can only choose between Light and Dark so this Blue one comes as an extra. This feature was spotted in the latest nightly version 98.