Netlify CMS needs to authenticate to Github with OAuth. Github needs a server to talk to during the authentication process. If you’re hosting at Netlify, they take care of that. If not, you’re on your own, and the documentation is... imperfect. 😂
If you’re deploying to Cloudflare Pages this may be the code you’re looking for. It provides API endpoints for Github to talk to, running on Cloudflare Functions. Functions are kind of like Cloudflare Workers, but they run right from your Pages site.
This is based on d3v1an7/netlify-cms-oauth-cloudflare adapted for Cloudflare Functions.
Go to Settings -> Developer Settings and choose OAuth Apps. Create a new OAuth App. Name it anything you like. Homepage URL should be set to your site’s URL. Authorization callback URL is the important part; it, too, can be set to your homepage URL (like https://example.com). Other guides say to set this to the callback API endpoint, but the documented requirement is that the callback URL be a subdirectory of this URL, and using the callback URL itself didn’t work for me.
Github will give you a Client ID, and you can generate a Client Secret at this time. You will need both.
In your Cloudflare Pages project, go to Settings -> Environment variables. Add two environment variables, GITHUB_CLIENT_ID set to the client ID from above, and GITHUB_CLIENT_SECRET set to the secret.
Take the functions directory from this repo and add it to your project at the top level. NOTE: This means the actual top-level of the project itself, not the top level of your output directory. So, if you’re using Hugo, functions goes at the top of the project next to content and layouts and archetypes and so forth, not inside the static or assets directory.
If you haven’t already installed the Netlify CMS files, take the static/admin directory from this repo and add it to your static directory (for Hugo) or wherever you add files to be deployed as-is to your site. There are two files under admin, a stub HTML file that loads the CMS, and the CMS config file config.yml. You can change the name of the admin directory.
The config file included here is a starter file only; you need to set this up for your site, which is beyond the scope of these instructions. However, you must set base_url in this config to the URL of your site (like https://example.com). Setting auth_endpoint here is optional because the default path works for the way this repo is set up.
Publish your site and let Cloudflare build it. Go to /admin/ on your site, and you should see a Login with Github button, which should authenticate you to Github and launch the CMS.
I've included a dark-mode CSS file from Nats-ji/netlify-cms-dark-mode.