The error log that systemd is showing about home-manager-username.service has more than what appears. systemd only shows the last 10 lines of the log of the service that failed. Run journalctl --user --unit home-manager-username.service to get the full logs.
In any case, Home Manager seems to be complaining about seeing files in your home folder that is defined in your Home Manager configuration but does not match what is defined in the Nix code. Did you add any files in your home folder, not through editing Nix files but through something else like a text editor, when you were experimenting with styles? The full log from the above journalctl command would point to those extra files if they exist.