When I use a manifest to theme my win32 app, checkboxes don't get themed

My Win32 app (C code, not C++) uses a manifest to get theme styles applied to common controls. This mostly works, but for some reason checkboxes that I create using CreateWindow("Button", NULL, WS_CHILD|BS_CHECKBOX...) don't get themed. Radiobuttons, created the same way (except BS_RADIOBUTTON) do get themed. And the funny thing is that checkboxes on dialogs in my .rc file also do get themed.

Any idea what's missing?