How to make custom CSS responsive?

I have a website and a premium theme on it. My client wanted a specific strictly designed fronpage outlook. The theme was not able to meet my client's wishes. Since the theme support team was not willing to help I had to change CSS through a lot of googling, wpmu dev sugestions and my own blood sweat. I got the final outlook which satisfied my client.
But the page with custom CSS is not yet responsive.
How can I make my custom CSS responsive?
Thank you.