When setting the text color for an invalid alias, use the Qt stylesheeet
mechanism rather than QPalette, so that it works correctly with
an applied QSS stylesheeet. Also attempt to detect a darkmode stylesheet
and use lighter shade of red so that the alias is more legible. Finally,
instead of explicitly setting the text color to black when it's valid,
reset to the original stylesheet (usually an empty string). This ensures
that in a dark stylesheet the text color is legible.
Fixes#0004803.
42e0cf0c8a