Change CommandMsg color from purple to gray

The intense purple stands out a lot more than it probably needs to.
Let's switch that to a gray color.

No settings migration logic included as custom colors are an
all-or-nothing approach.  Due to 'default.qss' using a system
palette color, there's no easy way to upgrade without changing
existing colors or loading all of Qt's color palettes in startup.
For example, palette(text) is dependent upon the system theme:
ChatLine {
  foreground: palette(text);
}