RFR: 8340203: Link color is hard to distinguish from text color in API documentation

[Hannes Wallnöfer]

Please review a CSS-only change to make it easier to recognize links in API documentation with the default stylesheet. 

This subtle change makes the link color a bit lighter and more saturated and normal text a bit darker. The new link color is `#437291` which is the same color used for links on [openjdk.org](https://openjdk.org/) and other OpenJDK sites. 

The table of contents and breadcrumb-navigation uses a slightly darker color closer to the original one, which works better with the gray background and the large amount of links.

The change also introduces CSS variables (aka custom properties) for the TOC colors which have been missing, and removes some redundant CSS rules for the TOC header.

I've uploaded API docs for `java.base` to compare the changes:

New colors: https://cr.openjdk.org/~hannesw/8340203/api.00/java.base/java/lang/Thread.html
Old colors: https://download.java.net/java/early_access/jdk24/docs/api/java.base/java/lang/Thread.html

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Commit messages:
 - JDK-8340203: Link color is hard to distinguish from text color in API documentation

Changes: https://git.openjdk.org/jdk/pull/21409/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21409&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340203
  Stats: 13 lines in 1 file changed: 6 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21409.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21409/head:pull/21409

PR: https://git.openjdk.org/jdk/pull/21409