Theme is styles an application to make it look appealing, attractive, and suit your tastes or needs.

Flutter’s MaterialApp comes with a default light blue theme.

Using Custom Theme

//Custom Theme in Flutter

    fontFamily: 'Roboto',
    brightness:     Brightness.light,
    primaryColor:   Color(#003c7e),
    textTheme: TextTheme( ... ),
headline:    TextStyle(fontSize: 36px, ... ),
    title:       TextStyle(fontSize: 24px, ... ),
    body:        TextStyle(fontSize: 12px, ... ),

Using Theme in Specific Widget
    color: Theme.of(context).primaryColor,    
    child: Text('custom theme', 
                 style: Theme.of(context).textTheme.body