We don't recommend editing the theme or plugin files directly, not via SSH/SFTP or the WordPress file editor. The problem in editing a theme or plugin directly is that when a new version is released, it will be updated over the existing files and all the modifications will be lost.
Editing Theme Style
We recommend making small CSS changes to the theme's Custom CSS field, or using a plugin made for that purpose, for example this Simple Header and Footer -plugin. Using a plugin makes sure that the changes are not lost when WordPress and its plugins or themes eventually updates.
Editing Theme
If bigger modifications to the site theme are needed, the best practice is to make a child theme of the main theme as explained in the official documentation in wordpress.org.
Preventing Theme or Plugin Updates
Another method of editing the site theme or plugin temporarily, is to copy the theme directory and rename it. For plugins you have to change the version number to something artificially high such as 999.
Warning!
This method also prevents security updates to be installed for the theme/plugin in question.