Challenge 3: Dark Mode Colors — Solution

// res/values/colors.xml — light mode (default)
<resources>
    <color name="background">#FFFFFF</color>
    <color name="text_primary">#1A1A1A</color>
</resources>

// res/values-night/colors.xml — dark mode (same names, inverted values)
<resources>
    <color name="background">#121212</color>
    <color name="text_primary">#E6E6E6</color>
</resources>

// activity_main.xml — references @color/ once, works for both modes
<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/background">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@color/text_primary"
        android:text="Hello, dark mode!" />

</LinearLayout>

Testing steps:
1. Run the app with the device/emulator in light mode — background
   should be near-white, text near-black.
2. Toggle system dark mode (emulator settings, or the quick-settings
   tile) without restarting the app.
3. The background and text colors should switch to the values-night
   versions automatically, with no code change and (in most cases) no
   app restart required.

Notes:
- Both colors.xml files define colors with the EXACT SAME names
  (background, text_primary) — the layout XML never references
  "values" or "values-night" directly, it just says @color/background,
  and Android resolves which file's definition applies based on the
  active system setting.
- If values-night/colors.xml were missing a color that values/colors.xml
  defines, Android falls back to the light-mode (default values/)
  definition for that specific color — values-night only needs to
  override the colors that actually differ.
- This only works because the app's theme has a DayNight parent
  (Theme.MaterialComponents.DayNight, from Challenge 2) — without that,
  values-night resources would simply never be selected.