chrisfrazier77 on "File permissions problem"

hello, I am pretty new to using wordpress. I am currently experiencing a problem with my woocommerce theme "mystile". On the Header.php, footer.php, and custom.css pages it has this message wear the "update file" button usually is: "You need to make this file writable before you can save your changes. See the Codex for more information". So I went to my directory using filezilla and tried to change the permissions of these files, but everytime I click okay, it gives me an error saying: "Error: set attrs for /wp-content/themes/mystile/custom.css: permission denied" How can i change the permissions so I can access these files?
also I am the administrator and am using woocommerce version 2.0.19.