8342705: Add dark mode for docs

[hns]

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 (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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Progress

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Issue

• JDK-8342705: Add dark mode for docs (Enhancement - P4)

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