> Does Aperture also work in DNN9? bootstrap 5?

Aperture has no frameworks (so no Bootstrap), its pure (fairly) modern CSS. It demonstrates heavy use of CSS variables, manual font loading in pages, and has a cool (pre-oklch()) color definitions. 

And no on DNN 9 also... I think Aperture is using DNN 10 features for CSS Variables. In PersonaBar, Manage/Styles you can see the new UI for editing the base css (only colors so far). These variables are attached to the page via:

(see your page source). They will be missing in DNN 9, but probably easy to wedge in if you know how such things work. Again, not sure, but I think Aperture needs these? Though it would be easy and trivial to get them copied and loading in DNN 9 (IMHO).

I do NOT think it was intended as a base or starting point for Themers to build from, but I could be wrong. The main goal, which I think it did well, was modernize DNN's "OOBE" (out of box experience). Also, its sample DDR menu does not go more than 2 levels deep.

Aperture can be used as a base starting point for theme development. The build system is very quick and efficient. The styles defined in the theme have fallbacks should the CSS variables not exist. So, I don't see why it wouldn't work on previous versions of DNN. That said, you should always strive to be on the latest version of DNN for many reasons. 😊

I think the responses explain how you can look at the Aperture theme. If you are comfortable with css variables, this theme might be for you.
If you want BS5 and open source, you can take a look at https://2shine.org/ or https://www.nvquicktheme.com/. Both need a bit of work to get going like installing yarn or ability to minify.
If this is not for you and you still want to use BS, you can consider DNNGo or EasyDNN which now released Buildora including a page builder with AI. TBH in the past I thought the EasyDNN themes were not as pretty as DNNGo but Buildora is pretty decent and worth to take a look.