How move file a11y-fontsize.css to child theme

You can use the filter wpa_fontsize_css to move the fontsize CSS to any location you wish. There’s an example of this here: https://github.com/joedolson/plugin-extensions/blob/master/wp-accessibility/custom-fontsize-css.php

Hello, I have not figured out how. Could you help me?

The GitHub URL I linked to is a sample plug-in. You’d want to install that information as a plug-in. You can upload it as FTP or zip it and upload it using the plug-in installer. You’d also need to edit it to provide the location for the file that you want; the given location would be a file named ‘fontsize.css’ in the root directory of a child theme.

Thanks! I just do these things that list?
1) create a file fontsize.css and I put it in my child theme.
2) take the file custom-fontsize-css.php, I put it in a folder and load it via ftp in the folder plugins.
3) the active and I can use the file fontsize.css to make my changes.

I only have one doubt. The folder I have to upload via FTP plugin, must contain only custom-fontsize-css.php and nothing else?

The plug-in can contain other things, but there’s no reason to include anything else; they’d just be ignored.