fix(explorer): keep toast descriptions readable in dark mode

Parent tracker: #436
Owning PR: #425
Depends on: #440 / PR #424

## Goal

Keep toast descriptions readable when the console uses its dark theme.

## Scope

- Correct the toast description color in dark mode.
- Preserve the existing light-theme behavior and toast semantics.

## Non-goals

- Redesigning the toast system.
- Restacking, merging, or otherwise changing PR state.

## Acceptance criteria

- Toast descriptions meet the existing visual contrast expectations in dark mode.
- Light-mode presentation remains unchanged.
- The focused presentation test or equivalent check covers the corrected style.

## Performance

No material performance impact is expected.

## Delivery

PR #425 remains based on PR #424 while the stack is open. After its predecessor merges, retarget it to staging and revalidate the exact head.

Start with the focused presentation test or equivalent check mentioned in the acceptance criteria, and review owning PR #425 together with predecessor PR #424. Verify the dark-mode toast description contrast while preserving light-mode presentation and existing toast semantics, then revalidate the exact head after the predecessor merges.

Written by the indexing model from the issue text.