As of the recent CSS update to quote markup and spoilers, quotes no longer have a colored background, while spoilers do:
This is a quote.
This is a spoiler.
When the spoiler is merely hovered over, the gray background remains in place, but when the spoiler is clicked (to continuously show the text without hovering), the gray background will disappear and it will look exactly like a quote. (This is more easily reproducible on desktop browsers; on mobile browsers, a touchscreen tap may just emulate a mouseover.)
While I wasn't sure whether this is an intentional feature or a bug, a CM said the following in response to a user complaint about this behavior:
Not so? Spoilers that are revealed should still have a grey background.
Therefore, it's a bug, and should be fixed.