It's about the preview panel. Ctrl +/- only makes sense for the markdown panel. The preview is basically just a web browser frame displaying some HTML. Maybe Ctrl +/- already works there. But that's something Internet Explorer has to make sure of.
It's just.. Markdown gets converted to HTML. After that conversion all the rules of the HTML world apply. That means: CSS for styling. There isn't much you can do about it. But it's actually the point of Markdown.