Seeking Suggestions for a SaaS "shell"

I'm sure there are lots of exceptions I'm not thinking about, but it seems to me that a typical SaaS startup has a core idea/service/product that requires some very specific software to deliver, but then also a bunch of code to run the website that handles all the stuff that has to be there, like signup forms, some kind of help interface, possibly some kind of configuration manager, possibly a dashboard.

My question: Are you using some kind of prebuilt website shell to get these "non-core" functions taken care of, or are you building that as part of the overall SaaS effort?

I mean, it seems crazy to spend time and effort on stuff that may need only a little tweaking to use if you get it prebuilt. But do you see that differently?

And if you built using a shell or framework for all the "other stuff," what did you use? What's been good/bad about doing it that way?