Update ThemePicker's dark theme colors

Bug: 129092060
Change-Id: I1d0af5495ed552451aae7899678426b364a366d4
diff --git a/res/layout/fragment_theme_picker.xml b/res/layout/fragment_theme_picker.xml
index dbb633e..d38e67f 100644
--- a/res/layout/fragment_theme_picker.xml
+++ b/res/layout/fragment_theme_picker.xml
@@ -36,7 +36,7 @@
                 android:id="@+id/theme_preview_pager"
                 android:layout_width="match_parent"
                 android:layout_height="wrap_content"
-                android:background="@color/secondary_color"
+                android:background="@color/preview_pager_background"
                 app:layout_constrainedHeight="true"
                 app:layout_constraintBottom_toTopOf="@id/options_container"
                 app:layout_constraintEnd_toEndOf="parent"