Plone 6 is actually made of at least a few building blocks – the frontend, backend, and typically a proxy server, and often more – and there are various ways to get and use the pieces. I came to focus on building containers with the frontend and backend elements customized as I chose. I can run them (and a proxy server) using docker compose. That affords using docker volumes to mount filestorage and blobstorage from my filesystem so I can persist the storage across container versions and back it up, etc. Similarly, I use volumes to mount the LetsEncrypt certs for the proxy server. See comment 6 that I mention above for the docker-compose.yml file with these and other features.
Section #4 in my writeup details how to develop and package addons to include in the container build of step #3. That is how you can customize your styles and create new Volto functionality to add to your system.
That path is elaborate, more so because there are still some obstacles that need to be navigated. My writeup and some details in these follow-up notes describe what I think is a full path, including workarounds for the current obstacles. It hinges on using containers to install and deploy it all, which presents some overhead if you're not already acquainted with using containers (I wasn't). Getting acquainted with them, it seems clear to me that containers are an incredibly useful tool. I believe they're essential if you want to install, deploy, and/or customize Plone 6.