The best way to start is to reactivate a theme that you know works properly.
Now, file permissions determines who can access the different files. If your permissions are too secure, the you can't make any changes (even though it's your site) and if they're too unsecure then people (hackers, etc) can use this to their advantage.
You can change permissions by:
1) Going into your ftp program (I recommend FileZIlla)
2) Right Click on the offending folder (in your case it's layout.css
3) Click File Permissions or CHMOD or whatever it is for your program (google it if you're not sure).
4) Select the permissions that you want. It suggests 666 - so try that. 744 is pretty good.
Does this make sense to you? http://codex.wordpress.org/Changing_File_Permissions