i mean if you want to keep it very simple, base flarum is already as simple as it gets imo.
you can not change html if you dont know how to write an extension so there is only css and minimal js.
and if you want my honest opinion on creating themes for flarum as i was trying to learn in the last 3 years, here it is:
its not the easiest. i mean i was a mybb forum owner back in the day and without knowing anything backend, i could do my own themes with html/css and variables.
in flarum though, if you cannot achive what you desire with only css or minimum js you have to at least write an extension to extend/override flarum's own appereance. and build it and deploy it, if you want to change some basic things like "like button from thumbs up to a heart" and dont know anything you could try and change flarum build .dist file it will work, but whenever you update flarum it will be turn back to the original.