[texhax] White text on black background?

[Karl Berry]

    P.S. Is it still the case tthat LaTeX is just a set of macros on plain
    TeX? 

Yes.

    And if so, is my inference correct that anything you can do in
    LaTeX, can be done -- perhaps at the cost of reimplementing big
    chunks of LaTeX -- in plain TeX?

Yes.  As mentioned, there is miniltx.

Also, Eplain (built on plain) uses miniltx and can load the color (but
not xcolor) LaTeX package.
See http://tug.org/eplain/doc/eplain/Loading-LaTeX-packages.html and
subsections.  If digging into the exact specials to use doesn't appeal
to you, I surmise this is the easiest way to go.

Best,
Karl