refresh styles; update Asciidoctor & syntax to 1.5.0; tweaks

I made a few refinements to the styles to align with some of the changes I made shortly before 1.5.0 release. I also selected a color for headings that aligns with the stylesheets from Java 8. I also made the body font darker since I think anything less than 80% black is just telling readers we don't like them ;)

Note: I did not update or test the Java 6 stylesheet. Feel free to look at the chances I made an decide which ones to port.