Dark theme hyper-link text was dark blue on black background in JDT's
Javadoc. The cause is that JDT's CSS contains hard-coded colors for
the links.
Replace these hard-coded colors with the symbolic names
hyperlinkColor and activeHyperlinkColor in the stylesheets.

Change-Id: I5f1843fbf4768f24fb60017a8fba918ca576f5cd
Signed-off-by: Matthias Becker <ma.becker@sap.com>