The theme editor is not available on multisite installations. Changing this would require modifications to WordPress Core, that would need to be remade everytime you updated WordPress. I STRONGLY advise against this.
Also keep in mind that if one user edits a theme, everyone else using that theme gets the changes. There is only one copy. The editor also allows users to edit other themes, so users can modify other peoples sites on the network this way. It’s also a massive security risk, a maintenance nightmare, and deprives you of future development work.
Instead I suggest you:
- Add theme options and settings for controlling the themes look
- Theme customiser options so that they have a nice UI to change things around, colours, text options, etc
- If they must add CSS, a setting with a text area like wordpress.com so that they can add CSS for the frontend. This way you can use some common javascript libraries to make editing the CSS much nicer than the editor you’re trying to re-enable.
I’d also note that with the editor you desire, users can horribly break their websites. A single typo or error saved to functions.php can cripple all websites in your install using that theme. The same thing can happen to the entire network if it happens in a plugin, bringing down the entire multisite installation.