RFR: 8342705: Add dark mode for docs [v2]

[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.03/java.base/java/lang/package-summary.html) (module `java.base` only). 
> 
> Screenshots below show both themes and how to switch between them.
> 
> <img width="1155" height="743" alt="light-theme" src="https://github.com/user-attachments/assets/d3b5703d-67d0-4f70-b8e4-39de13b74600" />
> <img width="1155" height="742" alt="dark-theme" src="https://github.com/user-attachments/assets/ab6f479b-df94-42bf-9c87-eea3978f38f1" />

Hannes Wallnöfer has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:

 - Update search tag highlight colors
 - Update link colors
 - Merge branch 'master' into JDK-8342705
 - 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

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Changes: https://git.openjdk.org/jdk/pull/26185/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26185&range=01
  Stats: 647 lines in 18 files changed: 600 ins; 25 del; 22 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