Rounded corner Textfiled without border color

Create a transparent border:

      final border = OutlineInputBorder(
            borderRadius: BorderRadius.all(Radius.circular(90.0)),
            borderSide: BorderSide(
              color: Colors.transparent,
            )
            );

Another option is using :

borderSide: BorderSide.none

And use it in focusedBorder and border properties, also add a Theme to set the cursor and hint Colors:

    Theme(
                  data: Theme.of(context).copyWith(
                    cursorColor: Colors.red,
                    hintColor: Colors.transparent,
                  ),
                  child: TextFormField(
                    decoration: InputDecoration(
                        focusedBorder: border,
                        border: border,
                        prefixIcon: Icon(
                          Icons.person,
                          color: Colors.white,
                        ),
                        hintStyle: TextStyle(
                            color: Colors.white, fontFamily: "WorkSansLight"),
                        filled: true,
                        fillColor: Colors.white24,
                        hintText: 'Password'),
                  ),
                ),

Set:

borderSide: BorderSide.none,

As in:

   TextFormField(
            decoration: InputDecoration(
                prefixIcon: Icon(
                  Icons.person,
                  color: Colors.white,
                ),
                border: OutlineInputBorder(
                  // width: 0.0 produces a thin "hairline" border
                  borderRadius: BorderRadius.all(Radius.circular(90.0)),
                  borderSide: BorderSide.none,
                ),

                hintStyle: TextStyle(color: Colors.white,fontFamily: "WorkSansLight"),
                filled: true,
                fillColor: Colors.white24,
                hintText: 'Password'),
          ),