Make h3-h6 stand out

Currently, h3 is quite hard to visually recognize from normal paragraph text, and h4 is only negligibly larger than the text, making these almost impossible spot. As the headings should provide an easily visible separation into sections, this adds a bit of weight on heading levels 3 to 6, making them nicely visible and quite useful.

(Not that I'd want to use h6 for anything ( :D ), but h3 is still pretty okay for subsectioning larger list-ish howtos.)