Incorrect file permissions on your server may cause many issues. After uploading any files to your server in order to install themes or extensions you need to set the correct file permissions for all the uploaded files – the same permissions as for the default Magento files.
File permissions may differ depending on server – usually 755 for folders and 644 for files. In case of any doubts about permissions, please contact your hosting provider. Refer to this article for more details.