Bug 1736158 - Force light color-scheme for reftests. r=jgraham

Bug 1529323 changed prefers-color-scheme to follow the Firefox theme. This
means that dev-edition has a dark prefers-color-scheme by default, because it
uses the dark theme.

Reftests rely on light color-schemes among other things because about:blank
draws a dark background when the user prefers a dark color scheme if it has no
opener.

Differential Revision: https://phabricator.services.mozilla.com/D128676

testing/profiles/reftest/user.js

@@ -98,3 +98,5 @@ user_pref("dom.push.serverURL", "");
 user_pref("toolkit.telemetry.initDelay", 99999999);
 // Setting this pref to true for usercss reftests, since it relies on userContent.css
 user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", true);
+// Use a light color-scheme unless explicitly overriden.
+user_pref("layout.css.prefers-color-scheme.content-override", 1);

testing/profiles/web-platform/user.js

@@ -67,3 +67,5 @@ user_pref("browser.cache.offline.enable", true);
 // Enable blocking access to storage from tracking resources by default.
 // We don't want to run WPT using BEHAVIOR_REJECT_TRACKER_AND_PARTITION_FOREIGN (5 - aka Dynamic First Party Isolation) yet.
 user_pref("network.cookie.cookieBehavior", 4);
+// Force a light color scheme unless explicitly overriden by pref.
+user_pref("layout.css.prefers-color-scheme.content-override", 1);