Since I realized there was no reason not to and it could be potentially useful to others, I tossed the renderkit I use for the WrapBootstrap Ace theme up on GitHub:

https://github.com/jesse-gallagher/Miscellany

As implied by the fact that it's not even a top-level project in my own GitHub profile, there are caveats:

• The theme itself is not actually included. That's licensed stuff and you'd have to buy it yourself if you want to use it. Fortunately, it's dirt cheap for normal use.
• It's just a pair of Eclipse projects, the plugin and a feature. To use it, you'll have to import them into Eclipse (or Designer, probably) with an appropriate plugin development environment, copy in the files from the Ace theme to the right place, export the feature project, and add it to Designer and Domino, presumably through update sites
• Since it's not currently actually extending Bootstrap4XPages (though I did "borrow" a ton of the code, where attributed), it may not cover all of the same components that that project does.
• I make no guarantees about maintaining this forked version, since the "real" one with the assets included is in a private repository.
• I haven't added the theme to the xsp.properties editor via the handy new ability IBM added to the ExtLib yet. You'll have to name it manually, as "wrapbootstrap-ace-1.3" with "-skin1", "-skin2", and "-skin3" suffix variants.

Still, I figured it may be worthwhile both as a plugin directly and as an educational endeavor. I believe I cover a couple bases that Bootstrap4XPages doesn't and, since I'm writing for a predefined theme with a specific set of plugins and for my own needs, I was able to make more assumptions about how things should work. Some of those are a bit counter-intuitive (for example, a "bare word" image property on the layout linksbar (like image="dashboard") actually causes the theme to render a Font Awesome icon with that name), but mostly things should work like you'd expect. The theme contains no controls of its own.

So... have fun with it!