Use NSApplication effectiveAppearance to determine dark mode

[jan2000]

Meld does not correctly check dark mode, it should be checked against NSApplication effectiveAppearance. This way one can turn off dark mode per application. Basically the same bug as: https://bugzilla.mozilla.org/show_bug.cgi?id=1593390

Patch based on patch the patch attached to the bugzilla bug: https://phabricator.services.mozilla.com/D51947 (however missing the OnMojaveOrLater and bestMatchFromAppearancesWithNames part):

--- /Applications/Meld.app/Contents/Resources/meld-org  2020-06-04 20:00:10.000000000 +0200
+++ /Applications/Meld.app/Contents/Resources/meld  2020-06-04 20:00:54.000000000 +0200
@@ -44,9 +44,8 @@
 gtk_theme= os.environ.get('GTK_THEME')
 if gtk_theme is None:
     try:
-        from Foundation import NSUserDefaults
-        standardUserDefaults = NSUserDefaults.standardUserDefaults()
-        if standardUserDefaults.stringForKey_("AppleInterfaceStyle") == 'Dark':
+        from Cocoa import NSApplication
+        if NSApplication.sharedApplication().effectiveAppearance().name() == "NSAppearanceNameDarkAqua":
             os.environ['GTK_THEME'] = "Meld-Os-Catalina-gtk:dark"
         else:
             os.environ['GTK_THEME'] = "Meld-Os-Catalina-gtk:light"

With this above patch one can turn dark mode off for Meld:

defaults write org.gnome.meld NSRequiresAquaSystemAppearance -bool yes

Turn back on with:

defaults delete org.gnome.meld NSRequiresAquaSystemAppearance