For app users

I’ll tell you right away: this way won’t be useful if the accent color is important to you.

Open Registry Editor and find HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\DWM section. You’ll need the following parameters from it:

• ColorPrevalence: DWORD = 1
  This parameter allows to use accent color as title bar color.
• AccentColor and AccentColorInactive: DWORD = color in ABGR format (hexadecimal).
  Colors for active and inactive mode.

Here’s an example of registry file where color #000000 is used for active window title and #212121 is used for inactive one:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\DWM]
"ColorPrevalence"=dword:00000001
"AccentColor"=dword:ff000000
"AccentColorInactive"=dword:ff212121

For app coders

You’ll need DwmSetWindowAttribute function:

HRESULT DwmSetWindowAttribute(
       HWND    hwnd,        // window
       DWORD   dwAttribute, // attribute itself
  [in] LPCVOID pvAttribute, // value
       DWORD   cbAttribute  // value size in bytes
);

Attribute values are described by DWMWINDOWATTRIBUTE but we specifically interested in DWMWA_USE_IMMERSIVE_DARK_MODE. Documentation says it’s equal to 20, however until 20H1 version the value has been equal to 19 .

That’s how i’ve tried to implement autodetect of window frame theme in Ultimate++ :

  HRESULT (WINAPI *DwmSetWindowAttribute)(HWND hwnd, DWORD dwAttribute, LPCVOID pvAttribute, DWORD cbAttribute);
  DllFn(DwmSetWindowAttribute, "dwmapi.dll", "DwmSetWindowAttribute");
  if (DwmSetWindowAttribute) {
    BOOL useDarkTheme = IsDarkTheme(); 
    DwmSetWindowAttribute(
      top->hwnd, 20, /* 20 is DWMWINDOWATTRIBUTE::DWMWA_USE_IMMERSIVE_DARK_MODE */
      &useDarkTheme, sizeof(useDarkTheme));
  }