Most standard styles highlight theorems and definitions using italics. Now this is something I really would like to avoid in my thesis (and generally). Unfortunately, if I just remove this "feature", I am left with the problem that it isn't obvious anymore where a theorem ends and the normal body text continues. I am wondering whether there is a not-too-complicated way to make theorem environments visually like quote environments, i. e. greater horizontal offset. What should I do?
An additional question in case this is too easy: What if I don't just want a greater horizontal offset, but also a vertical line on the level of the normal offset going all the way down until the end of the theorem?
For the first question, you can define a new environment using the quote environment and some therem-like structure from, for example, the amsthm package; for the second question, the leftbar environment from the framed package can be useful. A little example that you can use as starting point: