The question (and suggested answer) are a little obscure, but what is being described is mutt's use of the default color feature of ncurses (or slang). If your mutt color scheme uses the word "default" for the foreground or background, then at runtime mutt will ask ncurses/slang to use the terminal's default color.
Whether in an application such as
mutt or in a multiplexer such as
screen, it is implemented by SGR 39 (foreground default color,
^[[39m) and SGR 49 (background default color,
^[[49m). The multiplexer will use the terminal's actual SGR 39/49 support if it exists.
Applications such as the compositor can (relatively) easily detect the default foreground and background colors assigned to the terminal window and manipulate the background color to simulate a transparent window. Not everyone likes the effect (and because it detracts from readability, has not been implemented in xterm).
Default colors have their uses separate from transparent terminals, and mutt was one of the first applications where the feature was used (the result of a collaboration with Liviu Daia). It is coupled with the terminfo
bce (back color erase) feature, which was originally not supported by GNU screen. That's been supported since 2002 (from the question it's unclear whether OP knew how to configure it).
If you want the background to be opaque, then
mutt without using the
screen without enabling its