Question

TMontCalendar seems to be a Windows wrapper so it can’t be affected by the new VCL Styles, do you know a solution for it ?

Answer 1

The TMonthCalendar is wrapper for the MONTHCAL_CLASS and as far i know this control doesn’t support owner draw, but provides the CalColors property which allow you to set the colors of the elements of the calendar, but this property only works when the themes is not enabled. So first you must call the SetWindowTheme function to disable the themes in the calendar and then you can set the colors to match with the vcl styles.

Something like this

uses
  Vcl.Styles,
  Vcl.Themes,
  uxTheme;

Procedure SetVclStylesColorsCalendar( MonthCalendar: TMonthCalendar);
Var
  LTextColor, LBackColor : TColor;
begin
   uxTheme.SetWindowTheme(MonthCalendar.Handle, '', '');//disable themes in the calendar
   MonthCalendar.AutoSize:=True;//remove border

   //get the vcl styles colors
   LTextColor:=StyleServices.GetSystemColor(clWindowText);
   LBackColor:=StyleServices.GetSystemColor(clWindow);

   //set the colors of the calendar
   MonthCalendar.CalColors.BackColor:=LBackColor;
   MonthCalendar.CalColors.MonthBackColor:=LBackColor;
   MonthCalendar.CalColors.TextColor:=LTextColor;
   MonthCalendar.CalColors.TitleBackColor:=LBackColor;
   MonthCalendar.CalColors.TitleTextColor:=LTextColor;
   MonthCalendar.CalColors.TrailingTextColor:=LTextColor;
end;

And the result will be this