Don't set margin: 0 in presentation mode paragraphs

Currently the default cryptpad CSS is setting margin: 0 for all paragraphs in presentation mode. By inspecting a presentation I see this:

    padding: 0;
    margin: 0;
    min-height: 0;
    min-width: 0;

Which makes successive paragraphs (nearly) overlap, among other things (squashed lists). Injecting custom CSS with p { margin: 1rem; } restores a reasonable appearance. Perhaps this should be made the default?

A more minor gripe is that the default font size in presentations is very large, I find font-size: 75% to be about the largest I would want. But that is personal taste and easily fixed.