MATH_SYMBOL are not word chars.

Otherwise we don't stop when we see a | or < or whatever when scanning
forward/backward words, and navigation in 99% of the worlds languages is
annoying.
Add support for -fx-bg-stroke.
Revamp isearch match faces.

Use the new stroke to put a red rectangle around all matches, and then a solid
red background with white border and white foreground for the active match.
High contrast!

Also mark said styles as important so they override existing styles (chiefly
text foreground for the active match, no one else uses bg fill or stroke much).