Classic Visual Editor issue

I have not been with WordPress too long so my question could be a bit dumb. Anyway, I have recently noticed that the classic visual editor does not work properly with some themes. What I mean is that when I was using Twenty Sixteen theme, the format in the editor was pretty much as in the preview. Now I have switched to Nisarg and the format in the editor is all over the place: the text takes the full width of the window, some photos are not resized. But it is still fine on the published webpage. It also looks fine in Guttenberg but I don’t use it because I don’t understand how to put photos inside text blocks.