This fixes the CSD colors of Firefox to be more legible, but not without some caveats due to upstream issues.
- The inactive titlebar foreground color is used regardless of window state.
- #MozillaGtkWidget does not have any visible way to discriminate between CSD on and CSD off unlike normal GTK windows, so any style changes apply to both.
BUG: 407639
FIXED-IN: 5.17.0