Correctly detect system dark mode in RenderedJsonField

[RoyLee1224]

Fixes: #54204

Root Cause

The UI now supports OS-level color mode detection. When a user relies on their OS settings to switch to dark mode (and has not manually toggled the theme in the UI), the state from the next-themes library is:

• theme: 'system'
• resolvedTheme: 'dark'

The previous implementation in RenderedJsonField was checking against the theme property. The check theme === 'dark' would fail, causing the component to incorrectly use a light-mode style, resulting in dark text on a dark background.

Solution

The solution is to consistently use resolvedTheme as the source of truth for the currently displayed theme.

RenderedJsonField.tsx has been updated to use useColorMode.

Additionally, I have confirmed that RenderedJsonField.tsx was the only remaining instance of using the useTheme hook.

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[ashb]

Could you give before and after screenshots by any chance please?

[RoyLee1224]

@ashb Sure!

darkmode_before.mp4

darkmode_after.mp4

[pierrejeambrun]

Thanks