TP: Minor style fixes

Use the correct color to inherit from dark/light theme and to get
a proper divider on the bottom nav.

Bug: 120560581
Change-Id: I5053f02ad46a1c4f22d93d59296d407f7fb0e408
diff --git a/res/layout/fragment_theme_picker.xml b/res/layout/fragment_theme_picker.xml
index ef40334..245a22c 100644
--- a/res/layout/fragment_theme_picker.xml
+++ b/res/layout/fragment_theme_picker.xml
@@ -19,7 +19,7 @@
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:orientation="vertical"
-    android:background="@color/category_picker_background_color">
+    android:background="?android:colorPrimary">
     <include layout="@layout/section_header"/>
 
     <com.android.customization.widget.PreviewPager