Flutter textfield background color on focus

It looks like it's caused by the splash effect on the textfield. I couldn't find a way to disable it for that specific widget but you can make it transparent by wrapping your TextField in a Theme widget and setting the splashColor to transparent:

Theme(  data: Theme.of(context).copyWith(splashColor: Colors.transparent),  child: TextField(    autofocus: false,    style: TextStyle(fontSize: 22.0, color: Color(0xFFbdc6cf)),    decoration: InputDecoration(      filled: true,      fillColor: Colors.white,      hintText: 'Username',      contentPadding:          const EdgeInsets.only(left: 14.0, bottom: 8.0, top: 8.0),      focusedBorder: OutlineInputBorder(        borderSide: BorderSide(color: Colors.white),        borderRadius: BorderRadius.circular(25.7),      ),      enabledBorder: UnderlineInputBorder(        borderSide: BorderSide(color: Colors.white),        borderRadius: BorderRadius.circular(25.7),      ),    ),  ),);

To do it you should set filled to true.

TextField(decoration: InputDecoration( fillColor: Colors.red, filled: true)),

use BorderSide.none as:

border: OutlineInputBorder(    borderRadius: BorderRadius.circular(10.0),    borderSide: BorderSide.none,),