I just want to style my registration form ie text field, dropdown list everything with my css. Anybody please tell me how it is possible?

I am assuming that you have made a custom registration form (module) and applying the styles through a separate .css file.
You can simply add the reference of the .css file in the .ascx file as you do in normal .NET projects and use the CssClass attribute of the dropdown, text field etc... to assign a particular style.