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 people 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.