RFR: 8342705: Add dark mode for docs

[Hannes Wallnöfer]

Please review an enhancement to add a dark theme to javadoc-generated API documentation. There is a new button in the top right corner that brings up a menu to switch between themes. Available options are "Light", "Dark", and "System Setting" (following the system-wide theme setting). The selected theme is stored and preserved across browser sessions.

Generated docs [can be viewed here](https://cr.openjdk.org/~hannesw/8342705/api.02/) (`java.base` module only). The docs were generated with syntax highlighting enabled in order to view the highlighting themes, although that is obviously not part of the change.

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Commit messages:
 - Finalize dark theme
 - Fix icon colors for dark theme
 - Improve theme support
 - Set light theme by default
 - Improve dark theme, implement theme switcher
 - Merge branch 'master' into JDK-8342705
 - JDK-8342705: Add dark mode for docs

Changes: https://git.openjdk.org/jdk/pull/26185/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342705
  Stats: 635 lines in 17 files changed: 593 ins; 25 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/26185.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26185/head:pull/26185

PR: https://git.openjdk.org/jdk/pull/26185