Dark theme

How to detect dark theme is OS?

Universal method

// by "dbannon" from Lazarus forum
function IsDarkTheme: boolean;
var
  Col : string
begin
  // if char 3, 5 and 7 are all 'A' or above, we are not in a DarkTheme
  Col := HexStr(qword(GetRGBColorResolvingParent()), 8);
  Result := (Col[3] < 'A') and (Col[5] < 'A') and (Col[7] < 'A');
end;

Windows

Example 1

// by "jwdietrich" from Lazarus forum
uses
  Windows, Win32Proc, Registry;
     
// IsDarkTheme: Detects if the Dark Theme (true) has been enabled or not (false)
function DarkTheme: boolean;
const
  KEYPATH = '\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize';
  KEYNAME = 'AppsUseLightTheme';
var
  LightKey: boolean;
  Registry: TRegistry;
begin
  Result := false;
  Registry := TRegistry.Create;
  try
    Registry.RootKey := HKEY_CURRENT_USER;
    if Registry.OpenKeyReadOnly(KEYPATH) then
      begin
        if Registry.ValueExists(KEYNAME) then
          LightKey := Registry.ReadBool(KEYNAME)
        else
          LightKey := true;
      end
    else
      LightKey := true;
      Result := not LightKey
  finally
    Registry.Free;
  end;
end;

macOS

Example 1

// by "jwdietrich" from Lazarus forum
uses
  MacOSAll;
     
function IsMinMacOS(Maj, Min: integer): boolean;
  { returns true, if this app runs on a macOS version as specified or newer }
var
  Major, Minor: SInt32;
  theError: SInt16;
begin
  result := false;
  theError := Gestalt(gestaltSystemVersionMajor, Major);
  if theError = 0 then
    theError := Gestalt(gestaltSystemVersionMinor, Minor);
  if theError = 0 then
    if (Major = Maj) and (Minor >= Min) or (Major > Maj) then
      Result := True;
end;
     
function MojaveOrNewer: boolean;
  { returns true, if this app runs on macOS X 10.14 Mojave or newer }
begin
  result := IsMinMacOS(10, 14);
end;
     
{ The following two functions were suggested by Hansaplast at https://forum.lazarus.freepascal.org/index.php/topic,43111.msg304366.html }
     
// Retrieve key's string value from user preferences. Result is encoded using NSStrToStr's default encoding.
function GetPrefString(const KeyName : string) : string;
begin
  Result := NSStringToString(NSUserDefaults.standardUserDefaults.stringForKey(NSStr(@KeyName[1])));
end;
     
// IsDarkTheme: Detects if the Dark Theme (true) has been enabled or not (false)
function DarkTheme: boolean;
begin
  Result := false;
  if MojaveOrNewer then
    Result := pos('DARK',UpperCase(GetPrefString('AppleInterfaceStyle'))) > 0;
end;

Example 2

Catalina added an "Auto" option where the computer switches between Light and Dark modes depending on time of day. The 'AppleInterfaceStyle' method apparently doesn't work if auto is enabled.

The solution is to get the NSApp.effectiveAppearance string which will be one of a number of values including 'NSAppearanceNameAqua' (standard light mode) and 'NSAppearanceNameDarkAqua' (standard dark mode). Google these to see the whole list which includes other light and dark modes with added contrast. The effectiveAppearance is correct in all modes including when auto mode kicks in.

Tested on Mojave, Catalina, Big Sur.

// by "Clover" from Lazarus forum
function IsMacDarkMode: Boolean;
var
  sMode: string;
begin
  //sMode := CFStringToStr( CFStringRef( NSUserDefaults.StandardUserDefaults.stringForKey( NSSTR('AppleInterfaceStyle') ))); // Doesn't work in auto mode
  sMode  := CFStringToStr( CFStringRef( NSApp.effectiveAppearance.name ));
  Result := Pos('Dark', sMode) > 0;
end;