As a partial answer to my own vague question, here is
a change set that is the beginning of one possible
solution.  It operates from the parent morphs rather
than having StringMorphs and TextMorphs look for the
background color of their parents.

If you like your browsers to have white-text on dark
backgrounds, as I do, this is a start that does work. 
However, the same concept needs to be applied to
buttons, PluggableTexts, the border, and the window
label.

Sadly, I do not have time to finish this right now. 
Comments are welcome.

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