resaving style.css – displays all its contents in presentation panel

Help,

I am new to WordPress and not very techy. I have tried to take any theme, make adjustments to the .css file (minor things like bg colours, text colours etc), then resave the style.css file and replace it the theme’s folder. The problem is every time I do this, although the changes appear in the theme, the actual content content of .css file is diplayed in the admin/presentation panel and sometimes in the actual site itself if the theme is listed on the sidebar. I am at a loss….don’t know why its happening and would appreciate any help.

Many thanks, H.