Modeled after the RFC text files, this is a very bare-bones theme that I use on my site https://xangelo.ca.
It has a very simple mobile layout and auto-swaps between light/dark mode based on user browser preferences. It also supports utteranc.es for comments. That's configurable by supplying valid configuration defined below.
In config.toml
[params]
subtitle = "Sub title of the site"
favicon = "/<path-to-site-icon>/logo.ico"
githubUrl = "https://github.com/<your-name>/<site-proj-name>/"
referrer = "always"
author = "..."
description = "..."
keywords = "..."
googleSiteVerification = "<google-site-verification-code>"
[params.utterances]
repo = "AngeloR/angelor.github.io"
issue_term="pathname"
label="comment"
theme="preferred-color-scheme"