wxComboBox adding hover effect

Question

Here is my code in C++ when initialize a frame in wxwidgets:

wxPanel *panel = new wxPanel(this, wxID_ANY);

combo_list = new wxComboBox(panel,ID_COMBOBOX);
combo_list->Append("Guitar");
combo_list->Append("Violin");

Connect(ID_COMBOBOX, wxEVT_COMMAND_COMBOBOX_SELECTED,
        wxCommandEventHandler(ComboBox::OnSelect));

However, there is no default hover effect, which is background color changing when you move mouse on a specific selection. And I find for an hour on google but still cannot find it. Can anybody told me how to make the hover effect?

I just want to implement two drop down lists. The content in the first one has been determined before the running the program. In the order one the content will only be determined after I choose a directory.