Hi, that's good question.
For the first you've certainly made a backup.
it's after all the media part you have to change back. The rest is template override, if you disable this, you get the old look.
I pesronally like the new look!
EDIT: No, I looked into the media part, there is only fontawesome. You must disable in [your template]/html/com_jem and
then you have the old look. But when you had other template overrides you get them back only with restoring the backup.
PHP 7.2.9 on different sites
JEM 2.3.0dev 2.0M and specials with Joomla! 3.9.3 stable on livesites
ja_simply, ja_purity_iii, multilingual DE EN IT FR
Just remove all files you copied into the template. I did not change the core JEM. All old layouts are already existing.
Remove from templates/<youTemplate>/html all folders containing jem (e.g. com_jem, mod_jem, mod_jem_teaser) the same folders you find in templates/yourTemplate/css. Also remove the folders.
Go to JoomlaRoot/media/com_jem and delete the FontAwesome folder.
A short question: Why do you want to go back?
I just want to improve the responsive jem.