Set background mode to transparent before calling owner-drawn user
handling.

diff --git a/dlls/comctl32/header.c b/dlls/comctl32/header.c
index e7493a8..351c8c9 100644
--- a/dlls/comctl32/header.c
+++ b/dlls/comctl32/header.c
@@ -178,8 +178,11 @@
 	dis.hDC        = hdc;
 	dis.rcItem     = r;
 	dis.itemData   = phdi->lParam;
+        oldBkMode = SetBkMode(hdc, TRANSPARENT);
 	SendMessageA (GetParent (hwnd), WM_DRAWITEM,
 			(WPARAM)dis.CtlID, (LPARAM)&dis);
+        if (oldBkMode != TRANSPARENT)
+            SetBkMode(hdc, oldBkMode);
     }
     else {
         UINT uTextJustify = DT_LEFT;