RFR: 8367321: Fix CSS bugs in dark theme

[Hannes Wallnöfer]

Please review a change to fix various problems with the recently introduced dark theme for API docs:

 - Minimizes flickering when page is loaded with dark theme (best experienced on large pages such as "All Classes")
 - Fixes link color in table captions of inherited member summaries and class use pages ([before](https://download.java.net/java/early_access/jdk26/docs/api/java.base/java/lang/Exception.html#method-summary) / [after](https://cr.openjdk.org/~hannesw/8366942/api.00/java.base/java/lang/Exception.html#method-summary))
 - Fixes section color in Serialized Form page ([before](https://download.java.net/java/early_access/jdk26/docs/api/serialized-form.html) / [after](https://cr.openjdk.org/~hannesw/8366942/api.00/serialized-form.html))

The change also includes some JS and CSS refactoring/cleanup. 

Generated documentation was tested on Firefox, Chrome and Safari and [can be viewed here](https://cr.openjdk.org/~hannesw/8366942/api.00/java.base/module-summary.html) (module java.base only).

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Commit messages:
 - 8367321: Fix CSS bugs in dark theme

Changes: https://git.openjdk.org/jdk/pull/27191/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367321
  Stats: 69 lines in 4 files changed: 24 ins; 10 del; 35 mod
  Patch: https://git.openjdk.org/jdk/pull/27191.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27191/head:pull/27191

PR: https://git.openjdk.org/jdk/pull/27191