RFR: 8370612: Simplify implementation of dark theme

[Hannes Wallnöfer]

On Mon, 3 Nov 2025 08:44:28 GMT, Jonathan Lampérth <duke at openjdk.org> wrote:

> With this it is now more feasible to share dark/light mode icons via themed variables like --x-svg (instead of --x-svg-dark).

Thanks for the excellent suggestion, I updated the PR. 

I also made some further tweaks to TOC button styles:

 - Change hover border color to the one intended for the purpose (`--button-border-color`), which is more visible especially in dark theme
 - Move the positioning for the hide/show sidebar buttons into a separate ruleset that does not apply to the sort button
 - Tweak TOC size and position of hide/show buttons so that the hide button doesn't overlap the TOC (which moved down a bit because of the added sort button)

I uploaded the stylesheet of the [sample docs](https://cr.openjdk.org/~hannesw/8370612/api.00/java.base/java/lang/Thread.html#setUncaughtExceptionHandler(java.lang.Thread.UncaughtExceptionHandler)) in-place, you may have to shift-reload to fetch the new version of the stylesheet.

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PR Comment: https://git.openjdk.org/jdk/pull/28085#issuecomment-3480050871