RFR: 8342705: Add dark mode for docs [v3]

[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 incrementally with two additional commits since the last revision:

 - Move theme button to main navigation menu
 - Move theme button to sub-navigation bar

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26185/files
  - new: https://git.openjdk.org/jdk/pull/26185/files/dfb3b247..a12a273c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26185&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26185&range=01-02

  Stats: 203 lines in 10 files changed: 121 ins; 66 del; 16 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