Support custom color themes for the app UI

Feature request: allow importing/applying a custom color theme (e.g. a VS Code-style theme JSON with editor/terminal/UI color tokens) to personalize the app's appearance, similar to how editors support custom themes.

Example use case: a user wants to match the app's colors to another tool's palette (e.g. background, accent, syntax highlighting colors) by importing a theme JSON file rather than being limited to built-in light/dark options.

Would be great to support:

• Importing a theme JSON (colors + token colors)
• Applying it to chat UI, terminal canvas, and code blocks
• A way to preview/switch themes without restarting the app

Additionally, inline code mentions/spans in chat responses (e.g. file.ts or functionName) currently render in the same color as surrounding body text, making them hard to distinguish at a glance. A custom theme (or even a built-in style tweak) should give inline code its own distinct color/background so it's visually scannable, similar to how fenced code blocks are already set apart.

No files, tests, or entry points are identified. Start by mapping the theme import, preview, and application surfaces for the chat UI, terminal canvas, and code blocks, including inline code spans. Done means a theme JSON can be imported, switched without restarting, applied across those surfaces, and inline code is visually distinct.