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

[Hannes Wallnöfer]

On Fri, 25 Jul 2025 16:31:11 GMT, Hannes Wallnöfer <hannesw at openjdk.org> wrote:

>> 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.05/java.base/java/lang/package-summary.html) (iteration 5, module `java.base` only). 
>> 
>> Screenshots below show both themes and how to switch between them.
>> 
>> <img width="1217" height="658" alt="light-theme-2" src="https://github.com/user-attachments/assets/6f12dff4-b698-45f8-9b1c-14ab19a41664" />
>> <img width="1216" height="654" alt="dark-theme-2" src="https://github.com/user-attachments/assets/ff0384ea-d098-4582-95fc-0e84a7c6ed78" />
>
> 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

Another iteration on this PR, moving the theme switcher icon in line with the main navigation links in the top navigation bar. 

I updated screenshots and [demo docs](https://cr.openjdk.org/~hannesw/8342705/api.05/java.base/java/lang/package-summary.html).

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PR Comment: https://git.openjdk.org/jdk/pull/26185#issuecomment-3119367230