Theme author help needed

You shouldn’t be editing the theme files anyway. If the theme gets updated because of feature enhancements, bug fixes, or security patches, or if the theme has to be updated because of a change to the WordPress core, then your changes will be lost. Instead, either create a child theme or use a CSS plugin like Jetpack or Custom CSS Manager if you’re just making CSS changes.

but what CSS changes exactly should I make?

It depends on what you are trying to change. I don’t see anything in your post above that says what changes you are trying to make. For instance, if you want to make all of your post titles blue, then you would add this rule by going to Appearance → Edit CSS (after you activate the Custom CSS option by going to Jetpack → Settings):

.entry-title {
   color: #0000ff;
}

If you don’t want to change anything, then why would you need the Appearance → Editor?

Some hosts will disable the editor to prevent users from accidentally making changes to a theme (as I mentioned earlier, you generally do not want to make changes to theme files because they will get overwritten the next time the theme is updated). Check your wp-config.php file and see if this line is somewhere inside of it:

define('DISALLOW_FILE_EDIT', true);

You should also disable all plugins and see if that works. Some security plugins will disable the editor as well.

Remove that line from the wp-config.php file and then you should be OK.