BCGSuite for MFC
BCGCBPRODLLEXPORT UINT BCGM_ON_DRAW_SCROLLBAR

The framework sends BCGM_ON_DRAW_SCROLLBAR registered message to the scroll bar's owner when the scroll bar's style is set to BCGP_SBSTYLE_OWNER_DRAW.

Parameters
wParamcontrol ID of scroll bar.
lParama pointer to BCGPSCROLLBAR_DRAW_INFO structure.
Returns
The return value is ignored.

You should process this message to implement owner draw scrollbars.