本文整理自网络，侵删。
解决办法1：把自带 style 的 client 项关闭
解决办法2：
网上一搜，Stack Overflow 给出了方案――去掉自动绘制 style，去提取 style 的相关元素来设置 CalColors 属性。
尝试一下，效果基本能接受了。记在这里备查。
http://stackoverflow.com/questions/10335310/style-properties-for-tdatetimepicker
uses
Winapi.CommCtrl,
Vcl.Styles,
Vcl.Themes,
uxTheme;
Procedure SetVclStylesColorsCalendar( DateTimePicker: TDateTimePicker);
Var
LTextColor, LBackColor : TColor;
begin
uxTheme.SetWindowTheme(DateTimePicker.Handle, '', '');//disable themes in the calendar
//get the vcl styles colors
LTextColor:=StyleServices.GetSystemColor(clWindowText);
LBackColor:=StyleServices.GetSystemColor(clWindow);
DateTimePicker.Color:=LBackColor;
//set the colors of the calendar
DateTimePicker.CalColors.BackColor:=LBackColor;
DateTimePicker.CalColors.MonthBackColor:=LBackColor;
DateTimePicker.CalColors.TextColor:=LTextColor;
DateTimePicker.CalColors.TitleBackColor:=LBackColor;
DateTimePicker.CalColors.TitleTextColor:=LTextColor;
DateTimePicker.CalColors.TrailingTextColor:=LTextColor;
end;
procedure TForm2.DateTimePicker1DropDown(Sender: TObject);
var
hwnd: WinAPi.Windows.HWND;
begin
hwnd := SendMessage(TDateTimePicker(Sender).Handle, DTM_GETMONTHCAL, 0,0);
uxTheme.SetWindowTheme(hwnd, '', '');//disable themes in the drop down window
end;
procedure TForm2.FormCreate(Sender: TObject);
begin
SetVclStylesColorsCalendar(DateTimePicker1);
end;
来源：https://www.cnblogs.com/codingnote/archive/2014/10/24/4047264.html
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