cmd/compile: add dark mode functionality to CFGs in the ssa.html output

cmd/compile: add dark mode functionality to CFGs in the ssa.html output

add dark mode to CFGs in the ssa.html output by targeting individual
parts of each svg and applying dark mode styles to the stroke & fill.

Fixes #37767

Change-Id: Ic867e161c6837c26d9d735ea02bc94fdb56102f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/222877
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>