Suggested compromise
The major design goal of this change appears to be to remove the "overpowering" background:
Historically, we had a bit of feedback that the yellow backgrounds were overpowering, reducing the contrast of the quote itself. The yellow border also lacks contrast with the background.
Yet, according to users, differentiation is also important:
Having that rules text highlighted both distinctly and differently to the regular text is important, as it clarifies what text is in the author of the post's "voice" and what text is from the game system itself.
So, it looks like the solution is just to use a gentler color, and perhaps a border? (Probably only a border on the outermost blockquote)
With nested blockquotes, something like
