Challenge 2: A Custom App Theme — Solution

// res/values/colors.xml
<resources>
    <color name="brand_primary">#3DDC84</color>
    <color name="brand_on_primary">#0F1117</color>
</resources>

// res/values/themes.xml
<resources>
    <style name="Theme.MyFirstApp" parent="Theme.MaterialComponents.DayNight">
        <item name="colorPrimary">@color/brand_primary</item>
        <item name="colorOnPrimary">@color/brand_on_primary</item>
    </style>
</resources>

// AndroidManifest.xml — inside <application>
<application
    android:theme="@style/Theme.MyFirstApp"
    ... >
    ...
</application>

// activity_main.xml — an UNSTYLED default Button, no "style" attribute at all
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Default Button" />

Notes:
- Because AndroidManifest.xml's <application> tag sets android:theme to
  Theme.MyFirstApp, EVERY Activity in the app (and every View inside
  those Activities) inherits colorPrimary and colorOnPrimary
  automatically — no per-Activity or per-View theme reference needed.
- Running the app shows the plain, unstyled Button in activity_main.xml
  automatically tinted with brand_primary and its text in
  brand_on_primary — a standard Material Button reads colorPrimary from
  the active theme by default, which is the whole point of a theme
  versus Challenge 1's per-View style.
- parent="Theme.MaterialComponents.DayNight" (rather than a non-DayNight
  parent) is what makes this theme automatically participate in dark
  mode switching, setting up Challenge 3's values-night behavior to work
  correctly.