Yellow lines under Text Widgets in Flutter?

The problem is having a Scaffold or not.
Scaffold is a helper for Material apps (AppBar, Drawer, that sort of stuff). But you’re not forced to use Material.

What you’re missing is an instance of DefaultTextStyle as a parent:

DefaultTextStyle(
  style: TextStyle(...),
  child: Text('Hello world'),
)

Various widgets add one to change the default text theme, such as Scaffold, Dialog, AppBar, ListTile, …

It’s DefaultTextStyle that allows your app-bar title to be bold by default for example.