Themisbigender is a term under the themisgender umbrella, which refers to an individual who feels like themisfeminine and themismasculine at the same time or feels like 1-49% bigender, but without the agender part like libragender individuals do.
This term was coined by stickandst0n3s on the 28th of december 2021 on Twitter The flag doesnt really represent anything, it is a mix of the themisfeminine and themismasculine flags.