first, don't use the builtin editor , those changes are only temporary and will go away the first time you refresh the style components or purge the cache.
second, https://www.phpbb.com/kb/article/tools- ... ise-phpbb/
you don't edit "in your ftp". some ftp clients allow you to set your text editor from the options, but you are using the text editor , not the ftp.
the best thing to do is install a good text editor, use your ftp to download the files and edit them in the text editor.
upload back, refresh the style components from the admin panel/styles tab.
refresh your browser, done.
if you are going to be making changes it is usually best to change this in the admin panel:
on the left menu , click on load settings.
on the right, check yes for recompile stale style components.
now, your edits done in your text editor and uploaded to the server will be instantly available .