RFR: 8373679: Link color accessibility issue in dark theme

[Hannes Wallnöfer]

Please review a workaround for an accessibility issue in the JavaDoc dark theme. The color used for links in the dark theme does not have enough contrast from normal text according to WCAG 2. The workaround adds underlining to links in mixed content in the dark theme to make links easier to recognize. The change is implemented with a single nested CSS rule in the block that defines the dark theme settings.

Generated documentation can be viewed here: 
https://cr.openjdk.org/~hannesw/8373679/api.00/java.base/java/lang/package-summary.html
(java.base module only, change affects dark modse only)

Compliance of generated docs with WCAG 2 contrast requirements was tested with various A11Y tools.

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Commit messages:
 - 8373679: Link color accessibility issue in dark theme

Changes: https://git.openjdk.org/jdk/pull/29364/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29364&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373679
  Stats: 8 lines in 2 files changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29364.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29364/head:pull/29364

PR: https://git.openjdk.org/jdk/pull/29364