feat: add theme field to Icon type for light/dark mode support

Add IconTheme type alias and theme field to the Icon class for parity
with the Go SDK. This allows servers to specify whether an icon is
designed for a light or dark background, enabling clients to select
the appropriate icon based on their UI theme.

Closes #1977

Author: gfortaine

src/mcp/types/__init__.py

@@ -70,6 +70,7 @@
     GetTaskRequestParams,
     GetTaskResult,
     Icon,
+    IconTheme,
     ImageContent,
     Implementation,
     IncludeContext,
@@ -275,6 +276,7 @@
     "BlobResourceContents",
     "EmbeddedResource",
     "Icon",
+    "IconTheme",
     "ImageContent",
     "ResourceContents",
     "ResourceLink",

src/mcp/types/_types.py

@@ -28,6 +28,8 @@
 ProgressToken = str | int
 Role = Literal["user", "assistant"]
 
+IconTheme = Literal["light", "dark"]
+
 TaskExecutionMode = Literal["forbidden", "optional", "required"]
 TASK_FORBIDDEN: Final[Literal["forbidden"]] = "forbidden"
 TASK_OPTIONAL: Final[Literal["optional"]] = "optional"
@@ -170,6 +172,10 @@ class Icon(MCPModel):
     sizes: list[str] | None = None
     """Optional list of strings specifying icon dimensions (e.g., ["48x48", "96x96"])."""
 
+    theme: IconTheme | None = None
+    """Optional theme specifier. 'light' indicates the icon is designed for a light
+    background, 'dark' indicates the icon is designed for a dark background."""
+
 
 class Implementation(BaseMetadata):
     """Describes the name and version of an MCP implementation."""