Normally, the thickness of a window is 4 pixels, which can be retrieved by GetSystemMetrics method. Can I change its value, for example 2 pixels?
Thank you very much!
Simple answer: No. Not for a specific window.
Complicated answer: The border is drawn as part of the "non-client" region of the window. This is all handled (under the hood) by the default processing (i.e. DefWindowProc), along with the caption, minimize, maximize buttons, etc. You can override this by handling the WM_NCPAINT message. You'll then be responsible for drawing the entire non-client area of your window. You'll also want to handle the WM_NCCALCSIZE message, so that Windows knows how much of the remaining space to give to your client area.
Alternatively, you can set the border style of your window to none. This will allow Windows to draw the caption for you, although it'll probably look slightly different. Unfortunately, by doing this, you lose the drag-to-resize functionality. For that, you'll need to handle the WM_NCHITTEST message.
来源：https://stackoverflow.com/questions/410720/can-i-change-the-thickness-of-the-border-of-a-window-with-mfc