The Custom HTML Prompt in KM does not offer a transparent/semitransparent background, as the Display Text Large action has, and as BetterTouchTool offers. With BTT, people can build some cool stuff like this:
If I were to use the Custom HTML Prompt to replace the Display Text Large function, I would probably use the BTT floating HTML because of the translucency background option. But, although I've been using the Custom HTML Prompt in many places, I would still prefer not to use the Custom HTML Prompt action if there are other easier ways.
Lastly, since @peternlewis "Noted" the request. I was just checking in to see if there have been any updates.