Windows 10 Aero Undocumented API https://blog.ysc3839.com/archives/2017/10/win10-aero-undocumented-api.html

example.c

HMODULE hUser = GetModuleHandleA("user32.dll");
if (hUser)
{
	pfnSetWindowCompositionAttribute setWindowCompositionAttribute = (pfnSetWindowCompositionAttribute)GetProcAddress(hUser, "SetWindowCompositionAttribute");
	if (setWindowCompositionAttribute)
	{
		ACCENT_POLICY accent = { ACCENT_ENABLE_BLURBEHIND, 0, 0, 0 };
		WINDOWCOMPOSITIONATTRIBDATA data;
		data.Attrib = WCA_ACCENT_POLICY;
		data.pvData = &accent;
		data.cbData = sizeof(accent);
		setWindowCompositionAttribute(hWnd, &data);
	}
}

structs.h

#pragma once

typedef enum _WINDOWCOMPOSITIONATTRIB
{
	WCA_UNDEFINED = 0,
	WCA_NCRENDERING_ENABLED = 1,
	WCA_NCRENDERING_POLICY = 2,
	WCA_TRANSITIONS_FORCEDISABLED = 3,
	WCA_ALLOW_NCPAINT = 4,
	WCA_CAPTION_BUTTON_BOUNDS = 5,
	WCA_NONCLIENT_RTL_LAYOUT = 6,
	WCA_FORCE_ICONIC_REPRESENTATION = 7,
	WCA_EXTENDED_FRAME_BOUNDS = 8,
	WCA_HAS_ICONIC_BITMAP = 9,
	WCA_THEME_ATTRIBUTES = 10,
	WCA_NCRENDERING_EXILED = 11,
	WCA_NCADORNMENTINFO = 12,
	WCA_EXCLUDED_FROM_LIVEPREVIEW = 13,
	WCA_VIDEO_OVERLAY_ACTIVE = 14,
	WCA_FORCE_ACTIVEWINDOW_APPEARANCE = 15,
	WCA_DISALLOW_PEEK = 16,
	WCA_CLOAK = 17,
	WCA_CLOAKED = 18,
	WCA_ACCENT_POLICY = 19,
	WCA_FREEZE_REPRESENTATION = 20,
	WCA_EVER_UNCLOAKED = 21,
	WCA_VISUAL_OWNER = 22,
	WCA_HOLOGRAPHIC = 23,
	WCA_EXCLUDED_FROM_DDA = 24,
	WCA_PASSIVEUPDATEMODE = 25,
	WCA_USEDARKMODECOLORS = 26,
	WCA_CORNER_STYLE = 27,
	WCA_PART_COLOR = 28,
	WCA_DISABLE_MOVESIZE_FEEDBACK = 29,
	WCA_LAST = 30
} WINDOWCOMPOSITIONATTRIB;

typedef struct _WINDOWCOMPOSITIONATTRIBDATA
{
	WINDOWCOMPOSITIONATTRIB Attrib;
	PVOID pvData;
	SIZE_T cbData;
} WINDOWCOMPOSITIONATTRIBDATA;

typedef enum _ACCENT_STATE
{
	ACCENT_DISABLED = 0,
	ACCENT_ENABLE_GRADIENT = 1,
	ACCENT_ENABLE_TRANSPARENTGRADIENT = 2,
	ACCENT_ENABLE_BLURBEHIND = 3,
	ACCENT_ENABLE_ACRYLICBLURBEHIND = 4, // RS4 1803
	ACCENT_ENABLE_HOSTBACKDROP = 5, // RS5 1809
 	ACCENT_INVALID_STATE = 6
} ACCENT_STATE;

typedef struct _ACCENT_POLICY
{
	ACCENT_STATE AccentState;
	DWORD AccentFlags;
	DWORD GradientColor;
	DWORD AnimationId;
} ACCENT_POLICY;

typedef BOOL (WINAPI *pfnGetWindowCompositionAttribute)(HWND, WINDOWCOMPOSITIONATTRIBDATA*);

typedef BOOL (WINAPI *pfnSetWindowCompositionAttribute)(HWND, WINDOWCOMPOSITIONATTRIBDATA*);

@like-me there is no public documentation on this which is why this gist exists :)