I'd like to display a grey rectangle behind text, so that certain parts of the text are more recognisable. I will do this to highlight problems' body.
I found various packages, like
tcolorbox but I feel they're too complicated for what I'm trying to achieve.
Ideally it would be a simple grey rectangle, just like what you get when you add a quote on Math.SE:
The special left border isn't even necessary. What's the simplest way to achieve this?