fix(dark-mode): indicate dark color-scheme for native browser controls

[yogeshwaran-c]

What

Adds color-scheme: dark to the html.dark-mode selector in _dark-mode.scss so that UA-rendered controls (scrollbars, form inputs, date/color pickers, etc.) pick up the dark styling automatically when Swagger UI is in dark mode.

Why

Swagger UI ships its own dark theme via html.dark-mode, but the browser does not know about it — native widgets continue to render as light by default. Users currently have to work around this by injecting <meta name="color-scheme" content="light dark"> in their host HTML template.

Setting the CSS color-scheme property on the dark-mode root is the standard way to tell the UA to render its built-in controls in dark. Because it is scoped to html.dark-mode, the light theme is unaffected.

Closes #10718

Changes

• src/style/_dark-mode.scss: add color-scheme: dark to the html.dark-mode block.

Test plan

• Toggle Swagger UI into dark mode and confirm that native scrollbars, text inputs, and selects now render with the dark UA styling, matching the rest of the theme.
• Toggle back to light mode and confirm no regression — light-mode native controls are unchanged.