I think Obsidian is going to be just about the friendliest option you’re going to find when it comes to being both easy to customize and highly customizable. It strikes a really sweet middle ground, IMO.
The thing is, Markdown itself simplifies the process of making things such as checklists. And the editor adds to that ease, by adding another checklist item when you press Enter, for example.
When it comes to customizing colors and fonts, if you already have a third-party theme in place that you like, and you have the plugin Style Settings installed, many themes will allow you to tweak various aspects of the theme from the plugin’s options pane. But if you want to dive in further, you can open the .css file for the style, which you can navigate to through Obsidian’s settings.
That said, some themes are extremely complex and their CSS could be rather opaque for newbies, depending on what you’re trying to change. Others are far less complex.
In general, I’d say the HTML and/or CSS required to change colors or fonts is pretty simple anyways. Like, you could enclose some text in a span element and set that text’s font or color or both to something different. Done. No CSS file required; just use inline css (I know, that’s frowned upon by purists).
You likely will have to use some HTML or CSS to style some things, because that’s what Markdown and Obsidian are built on. But that in itself is not that difficult. It just depends on how far you want to go. And there are lots of resources for help if you decide to start getting ambitious.