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// t4a_IsThisDarkTheme: Detects if the Dark Theme (true) has been enabled or not (false)
function t4a_ThisIsDarkTheme:boolean;
begin
Result := t4a_ColorToGrayScale(clWindow) < t4a_ColorToGrayScale(clWindowText); // guess dark theme
end;
{ Convert a color to grayscale, observing perception }
function t4a_ColorToGrayScale(aColor:TColor):TColor;
var
GreyNumber:integer;
begin
Result:=ColorToRGB(aColor);
// Convert Color to Grayscale if not yet Grayscale
// Grayscale  = (0.3 * Red) + (0.59 * Green) + (0.11 * Blue)
// Alternative = (0.2126 * Red) + (0.7152 * Green) + (0.0722 * Blue)
if (Red(Result)<>Green(Result)) and (Red(Result)<>Blue(Result)) then
 begin
  GreyNumber:= ( ( MathRound(Red(Result)  * 0.3) +
           MathRound(Green(Result) * 0.59) +
           MathRound(Blue(Result) * 0.11) ) div 3);
  if GreyNumber<0 then
   GreyNumber:=0
  else if GreyNumber>255 then
   GreyNumber:=255;
   Result:=RGBToColor(GreyNumber,GreyNumber,GreyNumber);
 end;
end;
procedure TForm1.FormPaint(Sender: TObject);
begin
Label1.Caption := BoolToStr( t4a_ThisIsDarkTheme, 'Dark Theme', 'Light Theme' );
end;Â Â
Note: This can be done even shorter without the grayscale function:
Â
// t4a_IsThisDarkTheme: Detects if the Dark Theme (true) has been enabled or not (false)
function TForm1.t4a_ThisIsDarkTheme:boolean;
var
RGBWin, RGBText: TColor;
const
Rgray = 0.3;
Ggray = 0.59;
Bgray = 0.11;
begin
RGBWin := ColorToRGB(clWindow);
RGBText:= ColorToRGB(clWindowText);
Result := ((Red(RGBWin)*Rgray)+(Green(RGBWin)*Ggray)+(Blue(RGBWin)*Bgray)) < ((Red(RGBText)*Rgray)+(Green(RGBText)*Ggray)+(Blue(RGBText)*Bgray));
end;
And there is even a shorter method (thanks Alextp from the Lazarus forum!):
// by "Hansaplast" & "Alextp" from Lazarus forum function IsDarkTheme: boolean; function _Level(C: TColor): double; begin Result:= Red(C)*0.3 + Green(C)*0.59 + Blue(C)*0.11; end; begin Result:= _Level(ColorToRGB(clWindow)) < _Level(ColorToRGB(clWindowText)); end;
