Styling an TCustomControl descendant results in gray background labels - delphi

Question

In a following control I use TLabel as up and down buttons. When I choose "Cobalt XEMedia" as a default project style, these labels are drawn with a gray background.
"Windows", "Cobalt XEMedia" and "Obsidian":
Please help to draw the label background with the same color as the form (see pictures):
unit UI.UpDownEdit;
interface
uses
Vcl.Controls, Vcl.StdCtrls, System.Classes;
type
TUpDownEdit = class(TCustomControl)
private
_upButton: TLabel;
_downButton: TLabel;
_edit: TEdit;
_loop: Boolean;
_maxValue: Integer;
_minValue: Integer;
_minDigits: Byte;
procedure _downButtonClick(Sender: TObject);
procedure _upButtonClick(Sender: TObject);
procedure _editEnter(Sender: TObject);
procedure _setLoop(const Value: Boolean);
procedure _setMaxValue(const Value: Integer);
procedure _setMinValue(const Value: Integer);
function _getValue(): Integer;
procedure _checkRange;
procedure _valueToEdit(v: Integer);
function _constrainValue(v: Integer): Integer;
procedure _editKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
procedure _stepUp();
procedure _stepDown();
procedure _setMinDigits(const Value: Byte);
protected
procedure Resize(); override;
public
constructor Create(AOwner: TComponent); override;
destructor Destroy(); override;
published
property MinValue: Integer read _minValue write _setMinValue;
property MaxValue: Integer read _maxValue write _setMaxValue;
property Loop: Boolean read _loop write _setLoop;
property MinDigits: Byte read _minDigits write _setMinDigits;
end;
procedure Register();
implementation
uses
Vcl.Dialogs, System.SysUtils, System.UITypes, Winapi.Windows;
procedure Register();
begin
System.Classes.RegisterComponents('UI', [TUpDownEdit]);
end;
{ TUpDownEdit }
constructor TUpDownEdit.Create(AOwner: TComponent);
begin
inherited;
Width := 100;
Height := 100;
_minValue := 0;
_maxValue := 100;
_minDigits := 1;
_upButton := TLabel.Create(Self);
_upButton.Parent := Self;
_upButton.Align := alTop;
_upButton.Alignment := taCenter;
_upButton.Caption := '▲';
_upButton.Font.Size := 20;
_upButton.OnClick := _upButtonClick;
_edit := TEdit.Create(Self);
_edit.Parent := Self;
_edit.Align := alClient;
_edit.Font.Size := 20;
_edit.Alignment := taCenter;
_edit.TabOrder := 1;
_edit.OnEnter := _editEnter;
_edit.OnKeyDown := _editKeyDown;
_downButton := TLabel.Create(Self);
_downButton.Parent := Self;
_downButton.Align := alBottom;
_downButton.Alignment := taCenter;
_downButton.Caption := '▼';
_downButton.Font.Size := 20;
_downButton.OnClick := _downButtonClick;
_valueToEdit(0);
end;
destructor TUpDownEdit.Destroy();
begin
FreeAndNil(_upButton);
FreeAndNil(_downButton);
FreeAndNil(_edit);
inherited;
end;
procedure TUpDownEdit._editKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
case Key of
vkUp: begin Key := 0; _stepUp(); end;
vkDown: _stepDown();
vkRight:
begin
keybd_event(VK_TAB, 0, 0, 0);
keybd_event(VK_TAB, 0, KEYEVENTF_KEYUP, 0);
end;
vkLeft:
begin
keybd_event(VK_SHIFT, 0, 0, 0);
keybd_event(VK_TAB, 0, 0, 0);
keybd_event(VK_TAB, 0, KEYEVENTF_KEYUP, 0);
keybd_event(VK_SHIFT, 0, KEYEVENTF_KEYUP, 0);
end;
end;
end;
procedure TUpDownEdit.Resize();
begin
inherited;
_upButton.Height := ClientHeight div 3;
_downButton.Height := ClientHeight div 3;
end;
procedure TUpDownEdit._stepUp();
var
ev: Integer;
begin
ev := _getValue();
Inc(ev);
_valueToEdit(_constrainValue(ev));
end;
procedure TUpDownEdit._stepDown();
var
ev: Integer;
begin
ev := _getValue();
Dec(ev);
_valueToEdit(_constrainValue(ev));
end;
procedure TUpDownEdit._upButtonClick(Sender: TObject);
begin
_stepUp();
end;
procedure TUpDownEdit._downButtonClick(Sender: TObject);
begin
_stepDown();
end;
procedure TUpDownEdit._editEnter(Sender: TObject);
begin
//_edit.SelectAll();
end;
function TUpDownEdit._getValue(): Integer;
begin
if TryStrToInt(_edit.Text, Result) then Exit();
_valueToEdit(0);
Result := 0;
end;
procedure TUpDownEdit._valueToEdit(v: Integer);
begin
_edit.Text := Format('%.*d',[_minDigits, v]);
end;
procedure TUpDownEdit._setLoop(const Value: Boolean);
begin
_loop := Value;
_checkRange();
end;
procedure TUpDownEdit._setMaxValue(const Value: Integer);
begin
_maxValue := Value;
_checkRange();
end;
procedure TUpDownEdit._setMinDigits(const Value: Byte);
begin
_minDigits := Value;
if _minDigits < 1 then _minDigits := 1;
_checkRange();
end;
procedure TUpDownEdit._setMinValue(const Value: Integer);
begin
_minValue := Value;
_checkRange();
end;
function TUpDownEdit._constrainValue(v: Integer): Integer;
begin
if v < _minValue then if _loop then v := _maxValue else v := _minValue;
if v > _maxValue then if _loop then v := _minValue else v := _maxValue;
Result := v;
end;
procedure TUpDownEdit._checkRange();
begin
_valueToEdit(_constrainValue(_getValue()));
end;
end.