1). put 2 skindata on main form,
sd1.SkinFormtype=sfMainform,
sd2.SkinFormtype=sfOnlyThisForm

2). add code in sd1.onskinform event

procedure TForm1.sd1FormSkin(Sender: TObject; aName: String;
var DoSkin: Boolean);
begin
if aname='Form1' then doskin:=false; //mainform name
end;