Challenge 2: A Custom MaterialTheme — Solution

private val AppColorScheme = lightColorScheme(
    primary = Color(0xFF3DDC84),
    onPrimary = Color(0xFF0F1117)
)

@Composable
fun MyAppTheme(content: @Composable () -> Unit) {
    MaterialTheme(
        colorScheme = AppColorScheme,
        typography = Typography(),
        content = content
    )
}

@Composable
fun DemoScreen() {
    MyAppTheme {
        Column(modifier = Modifier.padding(16.dp)) {
            Text("Themed Screen")
            Button(onClick = { }) {
                Text("Click Me")
            }
        }
    }
}

Notes:
- Every composable nested inside MyAppTheme { } (Text, Button, and
  anything they themselves contain) can implicitly read
  MaterialTheme.colorScheme — a default Material Button reads
  colorScheme.primary for its background automatically, without being
  individually styled, exactly like an unstyled Button picked up
  colorPrimary from an XML theme back in Course 1 Chapter 8.
- lightColorScheme(...) only needs to override the specific colors that
  differ from Material's defaults (primary, onPrimary here) — every
  other color role (secondary, background, surface, etc.) falls back to
  Compose's built-in Material defaults automatically.
- DemoScreen must be wrapped in MyAppTheme (or MyAppTheme applied higher
  up, e.g. around setContent { } in MainActivity) for the custom colors
  to actually take effect — a composable outside any MaterialTheme
  wrapper falls back to Compose's own built-in default theme instead.