Documentation colors conflicting with system theme make text unreadable

Assistant appears to set text color, but not background color for the documentation it renders. This causes background color from the system theme to be used, which might conflict with the text color.

Hence if the user has a dark background/light foreground theme set, and the assistant colors the text black, the user sees black text on a dark background, which is at best difficult to read.

This bug first appeared with some 4.x version of Qt when the documentation style changed as well.