only use dark mode

components/useColorScheme.ts

@@ -1 +1,6 @@
-export { useColorScheme } from 'react-native';
+// The useColorScheme value is always either light or dark, but the built-in
+// type suggests that it can be null. This will not happen in practice, so this
+// makes it a bit easier to work with.
+export function useColorScheme(): 'light' | 'dark' {
+  return 'dark';
+}