Update stylesheet for non-API JDK documentation

Now that Dejavu web fonts are bundled with JDK API docs (JDK-8324774), the `jdk-default.css` stylesheet used for specs and tool documentation could be updated to include these fonts via a simple `@import` declaration.

Parts of `jdk-default.css` could also be updated to reflect the changes to `stylesheet.css` in JDK-8320458, such as using flexbox layout to vertically align navigation bar content.