Improve dark-theme contrast and interaction affordances in the devtools viewer

Background

The @ai-sdk/devtools viewer’s hardcoded dark palette has insufficient contrast, especially on information-dense traces. Grid lines and tooltips blend into surrounding surfaces, interactive controls resemble plain text, and expand/collapse controls appear on inconsistent sides of headers. These issues make traces unnecessarily slow to scan and manipulate.

Feature Request

Redesign the viewer’s dark theme with a higher-contrast palette that remains readable when large amounts of information are visible. Make grid lines clearly visible and give all interactive elements consistent, recognizable visual affordances. In particular, distinguish the “available tools” and “usage” controls from plain text. Standardize expand/collapse widgets on one side of headers throughout the viewer. Render tooltips with both a visible border and a background that is distinct from the page beneath them. This dark-theme improvement must be independently deliverable and must not depend on light-mode support.

Related Issues

Extracted from #16153

Start by locating the @ai-sdk/devtools viewer and its dark-theme palette, then inspect the grid lines, tooltips, available-tools and usage controls, and expand/collapse headers. Done means the dark theme is independently higher-contrast, interactive elements are recognizable, header widgets are consistent, and tooltips have distinct borders and backgrounds.