Improve appearance of boxes for links and target

Make the following changes to the boxes around unit links,
subunit links, alternate unit links, target text and target
character:
  1. Make rounded borders for all boxes.
  2. The height of each unit link was reduced to 21px in commit
     eff173ad8de296d950b5f52aa3dfe7582ddd93a9. Increase it back
     to the default, i.e. 24px.
  3. Make the borders around the links darker with a colour that
     is closer to lightcyan than blue.