best free code editor for someone who only edits theme files

agree on vs code. bigger thing though: if it's wordpress, make your edits in a child theme. edit the parent theme directly and the next theme update wipes the lot. and save a copy of any file before you change it, a missing semicolon in functions.php can take the whole site down.

VS Code. It sounds like overkill but it isn't, you can ignore 95% of it. Colour highlighting for CSS and PHP alone will save you from missing brackets, and there are SFTP extensions so you can edit files on the server and save straight up. If you want something lighter, Notepad++ is fine too.