Trackbar control

[Weasel]

How can I ensure that the background colour of my trackbar control is the same
colour as the background of the dialog box containing it ?

[malibor]

You need to handle WM_CTLCOLORSTATIC to update background color:
https://docs.microsoft.com/en-us/windows/win32/controls/wm-ctlcolorstatic

How to do this?

Following code sets trackbar background color to green:

#include <d2d1.h>
#include <Windows.h>

// Brush handle used by system to paint trackbar background
HBRUSH hCtlBrush = NULL;

// Color conversion function
ToColorRef(const D2D1::ColorF& color) noexcept
{
	const BYTE red = static_cast<BYTE>(255 * color.r);
	const BYTE green = static_cast<BYTE>(255 * color.g);
	const BYTE blue = static_cast<BYTE>(255 * color.b);

	return RGB(red, green, blue);
}

// Handler for WM_CTLCOLORSTATIC 
LRESULT OnCtlColorStatic(WPARAM wParam, LPARAM lParam) noexcept
{
	HDC hDCStatic = reinterpret_cast<HDC>(wParam);
	const COLORREF background = ToColorRef(D2D1::ColorF::Green);

	SetTextColor(hDCStatic, RGB(0, 0, 0)); // black text
	SetBkColor(hDCStatic, background); // green background

	if (hCtlBrush == NULL)
	{
		hCtlBrush = CreateSolidBrush(background);
	}

	return reinterpret_cast<LRESULT>(hCtlBrush );
}

// Don't forget to destroy brush at some point in your code
DeleteObject(hCtlBrush);

EDIT:

Btw. if your next issue is to learn current background color of a dialog then:

HWND hDlg = /* dialog handle */

HDC dialogDC = GetDC(hDlg);
COLORREF dialog_background = GetBkColor(dialogDC);

You then simply update above handler (line 21) as follows:
const COLORREF background = dialog_background;

And your trackbar should be of same color as dialog background.