91.2.1 was *also* using the configured Light Theme colours (for the colour of the "quote bar" in HTML messages).
What has happened on the update to 91.3.1 is that it is now using the Light background colours for the Dark Theme. (In 91.2.1 it was, correctly, using the Dark Theme colours).
The previous entry wasn't entirely correct.
91.2.1 was *also* using the configured Light Theme colours (for the colour of the "quote bar" in HTML messages).
What has happened on the update to 91.3.1 is that it is now using the Light background colours for the Dark Theme. (In 91.2.1 it was, correctly, using the Dark Theme colours).