I am a long-time and vocal critic of various technical failings (IMO) of the FT/IB system, but if the dark mode background is #333333, and someone writes a post that explicitly uses the tag "color=#333333" as the color of their text (you can see it if you click Quote on that message and then the Source button in the editor), I am not sure what you want the software to do

Granted, one proposal that seems reasonable at first, might be to automatically pick a different color when the color tag is the same as the background, sometimes people do it deliberately -- e.g. presuming "light mode" sometimes people will ask a trivia or secret question kind of thing and use the color tag to make the text white so it doesn't appear initially but shows up when you highlight it. From a usability standpoint it seems like a bad idea for software to change a color setting that a user explicitly specified -- you would get complaints the other way.