[gnome-tweak-tool] appearance: Always offer Adwaita and High Contrast GTK+ themes

[Jeremy Bicha <jbicha src gnome org>]

commit e9d3a670732eb711736e2cfc396c0b685bb40e93
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Fri Nov 3 13:38:15 2017 -0400

    appearance: Always offer Adwaita and High Contrast GTK+ themes
    
    since they're built into GTK+ 3

 gtweak/tweaks/tweak_group_appearance.py |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/gtweak/tweaks/tweak_group_appearance.py b/gtweak/tweaks/tweak_group_appearance.py
index ccadefc..7380c47 100644
--- a/gtweak/tweaks/tweak_group_appearance.py
+++ b/gtweak/tweaks/tweak_group_appearance.py
@@ -57,6 +57,12 @@ class GtkThemeSwitcher(GSettingsComboTweak):
                     os.path.exists(os.path.join(d, "gtk-2.0")) and \
                         (os.path.exists(os.path.join(d, "gtk-3.0")) or \
                          os.path.exists(os.path.join(d, "gtk-3.{}".format(gtk_ver)))))
+
+        if 'Adwaita' not in valid:
+            valid += ['Adwaita']
+        if 'HighContrast' not in valid:
+            valid += ['HighContrast']
+
         return valid
 
 class IconThemeSwitcher(GSettingsComboTweak):