style.css problem

Hi, i’ve been attempting to modify a theme that I didn’t make. Anyways I wanted to change the header font to a larger size and I modified the style.css file. I also tried modifing other portions of the file as well. After making changes to the file and uploading through ftp I didn’t like how it changed the theme. So I wanted to revert back to how it orginally looked so I uploaded an unmodifed style.css file. The problem is the theme remains the same and doesn’t change back to how it suppose to orginally look. So does the style.css modify any other file after its uploaded and the website is visited?