Hello,
Unfortunately I am not able to replicate the issue which you have reported. I placed multiple comboboxes on an aspx page with their theme set to "midnight". There was no issue with the color of the text both on the development machine as well as deployment server.
I have attached the sample application for you. Could you please try running it at your end and share your observations? Also if you still face any issues then you can either modify my sample or send me your sample so that I can debug it at my end.
Regards,
Sankalp