From what I understand you are worried about giving FTP access to someone on your server.
You can lock write access down a particular user and folder which would be your user. But that said there is not a lot to stop them having a look around at other files. There are things you can do against this:
in any development you have two options:
1 - using child theme. full separate development to add or rewrite some functions.
2 - using template and css overrides. partial css changes within core theme.
child theme is the easiest way.
if your development takes too long, during this time master theme can be upgraded many times to fix security and design bugs,...
Child themes go alongside a base theme.
If you are happy with the base theme then no need to install the child theme. However if the child theme contains something additional that you require then you need to install both the base theme and the child. And then from the admin apply the child theme as per dev docs
My suggestion is to use Layout Handle to update the current layout.
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
It' sometimes happen, that after run any command bin/magento your generated folder has changed ovnership to youruser:youruser, but should be youruser:www-data (for apache2 for example).
To fix that run in magento directory:
sudo chown -R youruser:www-data .
sudo usermod -a -G www-data youruser
after this you will never have this problem after ...
For fixed error Something went wrong while saving this configuration: Area is already set while saving theme configuration. You want create plugin for override Magento\Email\Model\AbstractTemplate.php file in custom module. And update setForcedArea() function.
File path: magento/app/code/Vendor/AreaConfigFix/registration.php