Fix private browsing color and make it toggleable

Author: K4sum1

browser/locales/en-US/browser/preferences/preferences.ftl

@@ -150,7 +150,7 @@ r3dfox-native-checkbox-desc = This controls whether the scrollbar, checkboxes, a
 r3dfox-theme =
     .label = Use r3dfox theme styles
 
-r3dfox-theme-desc = Currently this just enables or disables the red caption bar tint in private mode.
+r3dfox-theme-desc = Currently this just enables or disables the red colors in private mode.
 
 r3dfox-less-fog =
     .label = Less intense caption bar fog

browser/themes/shared/privatebrowsing/aboutPrivateBrowsing.css

@@ -8,7 +8,10 @@
 html.private {
   --in-content-page-color: white;
   --in-content-text-color: white;
-  --in-content-page-background: linear-gradient(to bottom, #480808 0%,#240103 93%,#200000 97%,#1E0000 100%);
+  --in-content-page-background: #25003e;
+  @media (-moz-bool-pref: "r3dfox.customizations.enabled") {
+    --in-content-page-background: linear-gradient(to bottom, #480808 0%,#240103 93%,#200000 97%,#1E0000 100%);
+  }
   --link-color: white;
   --link-color-hover: white;
   --link-color-active: white;

browser/themes/shared/tabs.css

@@ -46,6 +46,9 @@
        ThemeVariableMap.sys.mjs when the user has a theme that defines
        ntp_background. */
     --tabpanel-background-color: #25003e !important;
+    @media (-moz-bool-pref: "r3dfox.customizations.enabled") {
+      --tabpanel-background-color: linear-gradient(to bottom, #480808 0%,#240103 93%,#200000 97%,#1E0000 100%) !important;
+    }
 
     /* stylelint-disable-next-line media-query-no-invalid */
     @media (-moz-bool-pref: "browser.privatebrowsing.felt-privacy-v1") {