[MCP]: switch between light and dark color scheme

[lkiesow]

What's going on?

Motivation

Coding agents driving the Playwright MCP server can only test a UI in whatever color scheme
the browser happened to start in. Frontend work almost always needs both — an agent that
implements or reviews a dark theme currently has no way to see it.

Nothing on the MCP surface exposes the color scheme today:

• None of the ~70 tools touches it (browser_resize is the closest neighbour).
• There is no CLI flag and no PLAYWRIGHT_MCP_* env var for it.

The existing workarounds are all unsuitable for an agent mid-session:

• browser.contextOptions.colorScheme in the JSON config file — requires writing a config
  file and restarting the server, and is fixed for the whole session.
• --init-page calling page.emulateMedia({ colorScheme: 'dark' }) — same restart problem.
• browser_run_code_unsafe — arbitrary code execution, off by default for good reason, and
  not something an agent should reach for to flip a media query.

Playwright itself supports this fully via
page.emulateMedia() and
contextOptions.colorScheme, so this is purely about surfacing existing capability.

Proposal

1. A browser_emulate_media tool wrapping page.emulateMedia(), with all parameters
optional so omitted media features are left unchanged:

Parameter	Values
colorScheme	light, dark
reducedMotion	reduce, no-preference
forcedColors	active, none
contrast	more, no-preference
media	screen, print

Scope matches browser_resize: core capability (available by default), applies to the
current page only, since page.setViewportSize() is per-page in exactly the same way.
Mirroring emulateMedia rather than adding a color-scheme-only tool means reduced-motion,
forced-colors and print emulation come along without a second tool later — useful for
accessibility and print-stylesheet testing.

2. A --color-scheme <light|dark> CLI option (plus PLAYWRIGHT_MCP_COLOR_SCHEME),
mapping to contextOptions.colorScheme alongside the existing --viewport-size and
--user-agent handling, so a whole session can be started in dark mode without a config
file.

Example

browser_navigate      { "url": "http://localhost:3000" }
browser_take_screenshot
browser_emulate_media { "colorScheme": "dark" }
browser_take_screenshot

[pavelfeldman]

Your patch looks good, please feel free to reopen it as is. I'd probably have playwright-cli set- in the CLI instead of passing camelCase options though.