Hi,
I think you are using Telerik Dropdownlist control, if that is the case then apologies from my side.By mistake I had provided you css style for Telerik Combo box control.
Please find the css to change the fonts style in both controls
1) Telerik Dropdownlist control
2) Telerik Combo box control
I have attached screenshot of css applied to the above two controls.
Thanks,
A2H