A great graphic design tip for simple websites is stop using additional files. If you can't do it with a single HTML file, you're adding unnecessary complexity.
Maybe people enjoy aesthetics other than minimalist? Maybe some people enjoy complex things?
I hope we can agree that adding unnecessary complexity to software is bad, and that simplicity should be something to strive for.
Am not saying graphic design is nearly as (inherently) complex as computer engineering, but neither benefits from complicating things that don't need to be complicated.
You provide a good example with your reference manual. The diagrams will be clear, the fonts readable, they wont have jazzy designs etc etc etc.