How to avoid showing files contnet on website

I have a simple PHP website , I want to know is there any way to avoid users Access to the files in my website ? like css files and other things ?

For prevent to show my files in folder and directories I used htaccess file and put this code inside htaccess :

Options -Indexes

but some users can steel access to the files content like css codes. for exampe if an user try this url :

www.mysite.com/folder

this message will show for him :

Forbidden

 You don't have permission to access /css/ on this server.

but if users try this url :

www.mysite.com/folder/file.css

Then the css will apeere to him ...

I want to know how can i prevent this problem ? And avoid to opening files by users ?