ThemeData class

theme: ThemeData( primaryColor: Colors.blue, accentColor: Colors.green, textTheme: TextTheme(bodyText2: TextStyle(color: Colors.purple)), ),

and many other properties in this.

for whole app

theme: ThemeData.dark().copyWith(  // copywith for showing body text we
  primaryColor: Color(0xFF0a0d22),       can also take only themedata
  scaffoldBackgroundColor: Color(0xFF090c21),
),

For particular icon

floatingActionButton: Theme(
  data: ThemeData(
    accentColor: Colors.purple,
  ),
  child: FloatingActionButton(
    child: Icon(Icons.add),
  ),
),