diff --git a/src/generated/schema.json b/src/generated/schema.json
index 63cb3d3..047ad29 100644
--- a/src/generated/schema.json
+++ b/src/generated/schema.json
@@ -268,7 +268,7 @@
               "description": "CSS variables for theming the app.",
               "type": "object",
               "propertyNames": {
-                "description": "Style variables for theming MCP apps.",
+                "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
                 "anyOf": [
                   {
                     "type": "string",
@@ -417,8 +417,13 @@
                 ]
               },
               "additionalProperties": {
-                "description": "Style variables for theming MCP apps.",
-                "type": "string"
+                "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
+                "anyOf": [
+                  {
+                    "type": "string"
+                  },
+                  {}
+                ]
               }
             },
             "displayMode": {
@@ -711,7 +716,7 @@
           "description": "CSS variables for theming the app.",
           "type": "object",
           "propertyNames": {
-            "description": "Style variables for theming MCP apps.",
+            "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
             "anyOf": [
               {
                 "type": "string",
@@ -860,8 +865,13 @@
             ]
           },
           "additionalProperties": {
-            "description": "Style variables for theming MCP apps.",
-            "type": "string"
+            "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {}
+            ]
           }
         },
         "displayMode": {
@@ -1336,7 +1346,7 @@
               "description": "CSS variables for theming the app.",
               "type": "object",
               "propertyNames": {
-                "description": "Style variables for theming MCP apps.",
+                "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
                 "anyOf": [
                   {
                     "type": "string",
@@ -1485,8 +1495,13 @@
                 ]
               },
               "additionalProperties": {
-                "description": "Style variables for theming MCP apps.",
-                "type": "string"
+                "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
+                "anyOf": [
+                  {
+                    "type": "string"
+                  },
+                  {}
+                ]
               }
             },
             "displayMode": {
@@ -2341,10 +2356,10 @@
     },
     "McpUiStyles": {
       "$schema": "https://json-schema.org/draft/2020-12/schema",
-      "description": "Style variables for theming MCP apps.",
+      "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
       "type": "object",
       "propertyNames": {
-        "description": "Style variables for theming MCP apps.",
+        "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
         "anyOf": [
           {
             "type": "string",
@@ -2493,8 +2508,13 @@
         ]
       },
       "additionalProperties": {
-        "description": "Style variables for theming MCP apps.",
-        "type": "string"
+        "description": "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
+        "anyOf": [
+          {
+            "type": "string"
+          },
+          {}
+        ]
       }
     },
     "McpUiTheme": {
diff --git a/src/generated/schema.ts b/src/generated/schema.ts
index 6b804dc..07b82cf 100644
--- a/src/generated/schema.ts
+++ b/src/generated/schema.ts
@@ -70,15 +70,27 @@ export const McpUiStyleVariableKeySchema = z
 
 /**
  * @description Style variables for theming MCP apps.
+ *
+ * Individual style keys are optional - hosts may provide any subset of these values.
+ * Values are strings containing CSS values (colors, sizes, font stacks, etc.).
+ *
+ * Note: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`
+ * for compatibility with Zod schema generation. Both are functionally equivalent for validation.
  */
 export const McpUiStylesSchema = z
   .record(
     McpUiStyleVariableKeySchema.describe(
-      "Style variables for theming MCP apps.",
+      "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
     ),
-    z.string().describe("Style variables for theming MCP apps."),
+    z
+      .union([z.string(), z.undefined()])
+      .describe(
+        "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
+      ),
   )
-  .describe("Style variables for theming MCP apps.");
+  .describe(
+    "Style variables for theming MCP apps.\n\nIndividual style keys are optional - hosts may provide any subset of these values.\nValues are strings containing CSS values (colors, sizes, font stacks, etc.).\n\nNote: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`\nfor compatibility with Zod schema generation. Both are functionally equivalent for validation.",
+  );
 
 /**
  * @description Request to open an external URL in the host's default browser.
diff --git a/src/spec.types.ts b/src/spec.types.ts
index 0dbfb03..5450f79 100644
--- a/src/spec.types.ts
+++ b/src/spec.types.ts
@@ -91,8 +91,14 @@ export type McpUiStyleVariableKey =
 
 /**
  * @description Style variables for theming MCP apps.
+ *
+ * Individual style keys are optional - hosts may provide any subset of these values.
+ * Values are strings containing CSS values (colors, sizes, font stacks, etc.).
+ *
+ * Note: This type uses `Record<K, string | undefined>` rather than `Partial<Record<K, string>>`
+ * for compatibility with Zod schema generation. Both are functionally equivalent for validation.
  */
-export type McpUiStyles = Record<McpUiStyleVariableKey, string>;
+export type McpUiStyles = Record<McpUiStyleVariableKey, string | undefined>;
 
 /**
  * @description Request to open an external URL in the host's default browser.
diff --git a/src/styles.ts b/src/styles.ts
index 4544248..45957be 100644
--- a/src/styles.ts
+++ b/src/styles.ts
@@ -109,6 +109,8 @@ export function applyHostStyles(
   root: HTMLElement = document.documentElement,
 ): void {
   for (const [key, value] of Object.entries(styles)) {
-    root.style.setProperty(key, value);
+    if (value !== undefined) {
+      root.style.setProperty(key, value);
+    }
   }
 }