adds an override/exception pref for dark mode and RFP

patches/allow_dark_preference_with_rfp.patch

@@ -0,0 +1,29 @@
+diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
+index ce2cbca..60fda48 100644
+--- a/dom/base/Document.cpp
++++ b/dom/base/Document.cpp
+@@ -17084,6 +17084,7 @@ void Document::RemoveToplevelLoadingDocument(Document* aDoc) {
+ StylePrefersColorScheme Document::PrefersColorScheme(
+     IgnoreRFP aIgnoreRFP) const {
+   if (aIgnoreRFP == IgnoreRFP::No &&
++      !StaticPrefs::privacy_override_rfp_for_color_scheme() &&
+       nsContentUtils::ShouldResistFingerprinting(this)) {
+     return StylePrefersColorScheme::Light;
+   }
+diff --git a/modules/libref/init/StaticPrefList.yaml b/modules/libref/init/StaticPrefList.yaml
+index b4b2dc7..e65d43f 100644
+--- a/modules/libref/init/StaticPrefList.yaml
++++ b/modules/libref/init/StaticPrefList.yaml
+@@ -9863,6 +9863,12 @@
+   value: false
+   mirror: always
+ 
++# Allows overriding RFP for only the dark/light pref
++- name: privacy.override_rfp_for_color_scheme
++  type: bool
++  value: false
++  mirror: always
++
+ # We automatically decline canvas permission requests if they are not initiated
+ # from user input. Just in case that breaks something, we allow the user to
+ # revert this behavior with this obscure pref. We do not intend to support this