Update dark theme color to match mock

Mock: https://docs.google.com/presentation/d/15JadcGi5k1_0znUN_XdFpU7UxeXwxjK3Y7LxvhW5ETM/edit?ts=5e1cc430#slide=id.g85fff7c452_76_21
Screenshot:
https://screenshot.googleplex.com/ytBaVdCCto0.png
https://screenshot.googleplex.com/TppopvRTG2t.png
https://screenshot.googleplex.com/cndtG9eKopv.png
https://screenshot.googleplex.com/0zPK9qaTqrY.png
https://screenshot.googleplex.com/mMRCbvq8AYT.png

Test: Manually
Fixes: 158587080
Change-Id: I48a20b13e133467d6e30c9c94ae0594df48cd019
diff --git a/res/layout/fragment_theme_picker.xml b/res/layout/fragment_theme_picker.xml
index a773207..2e3a6bb 100644
--- a/res/layout/fragment_theme_picker.xml
+++ b/res/layout/fragment_theme_picker.xml
@@ -36,7 +36,7 @@
                 android:layout_height="0dp"
                 android:paddingTop="@dimen/preview_content_padding_top"
                 android:paddingBottom="@dimen/preview_content_padding_bottom"
-                android:background="@color/fullscreen_preview_background"
+                android:background="@color/preview_pager_background"
                 android:clipToPadding="false"
                 app:layout_constrainedHeight="true"
                 app:layout_constraintStart_toStartOf="parent"