This little draft demonstrates how to switch from a light theme, to a dark application title bar with AutoIT, using DWM api.

dark_theme_dwmapi.au3

; This little draft demonstrates how to switch from a light theme, to a dark application title bar with AutoIT, using DwmAPI.
; It will probably be useful to someone who wants to do this, without any headache.
; Because windows does not have any official documentation on this subject today :s

#include <GUIConstantsEx.au3>
#notrayicon

opt('guioneventmode', 1)

#cs ----------------------------------------------------------------------------
 Function    : is_app_dark_theme()
 Description : returns if the user has enabled the dark theme for applications in the Windows settings (0 on / 1 off)
               if OS too old (key does not exist) the key returns nothing, so function returns False
#ce ----------------------------------------------------------------------------
func is_app_dark_theme()
	return(regread('HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize', 'AppsUseLightTheme') == 0) ? True : False
endfunc

#cs ----------------------------------------------------------------------------
 Function    : set_dark_theme($hwnd, $dark_theme = True)
 Description : set to the handle, the attribute to define whether dark/light theme
#ce ----------------------------------------------------------------------------
func set_dark_theme($hwnd, $dark_theme = True)
	; before this build set to 19, otherwise set to 20, no thanks Windaube to document anything ❤️
	$DWMWA_USE_IMMERSIVE_DARK_MODE = (@osbuild <= 18985) ? 19 : 20
	$dark_theme = ($dark_theme == True) ? 1 : 0
	
	dllcall( _
        'dwmapi.dll', _
		'long',   'DwmSetWindowAttribute', _
		'hwnd',   $hwnd, _
		'dword',  $DWMWA_USE_IMMERSIVE_DARK_MODE, _
		'dword*', $dark_theme, _
		'dword',  4 _
	)
endfunc

; create gui
$gui_hwnd = guicreate('Tiny gui', 800, 450)
guisetonevent($GUI_EVENT_CLOSE, 'gui_close')

; if dark theme enabled for apps in windows settings, set dark theme to gui
if is_app_dark_theme() == True then
	set_dark_theme($gui_hwnd, True)
endif

; display gui
guisetstate(@sw_show)

while 1
	sleep(10)
wend

func gui_close()
	exit
endfunc