I don't have a style.css file because I created the site many years ago and I didn't know how to create one!
But now I need to have one to make my site mobile friendly.
From what I found out by searching online you need to place the codes in your css file. So Can I create just create a .css file just to place this code?
I don't have time to design a complete css file and the current design of my site is ok for me. I just want to make the site mobile friendly.

Thanks for your help.