It is possible only by modifications in the dhtmlxcombo.css:
dhx_combo_input - style of the combo input;
dhx_combo_box - style of the container with input and image;
dhx_combo_list - style of the list
It is possible only by modifications in the dhtmlxcombo.css:
dhx_combo_input - style of the combo input;
dhx_combo_box - style of the container with input and image;
dhx_combo_list - style of the list