diff --git a/tools/ui/src/lib/components/app/settings/SettingsChat/SettingsChat.svelte b/tools/ui/src/lib/components/app/settings/SettingsChat/SettingsChat.svelte
index d017fe20469..69a120b7cb6 100644
--- a/tools/ui/src/lib/components/app/settings/SettingsChat/SettingsChat.svelte
+++ b/tools/ui/src/lib/components/app/settings/SettingsChat/SettingsChat.svelte
@@ -75,9 +75,13 @@
 	}
 
 	function handleSave() {
-		if (localConfig.custom && typeof localConfig.custom === 'string' && localConfig.custom.trim()) {
+		if (
+			localConfig.customJson &&
+			typeof localConfig.customJson === 'string' &&
+			localConfig.customJson.trim()
+		) {
 			try {
-				JSON.parse(localConfig.custom);
+				JSON.parse(localConfig.customJson);
 			} catch (error) {
 				alert('Invalid JSON in custom parameters. Please check the format and try again.');
 				console.error(error);
diff --git a/tools/ui/src/lib/constants/settings-keys.ts b/tools/ui/src/lib/constants/settings-keys.ts
index 53243992fa2..7cdd2db7c4d 100644
--- a/tools/ui/src/lib/constants/settings-keys.ts
+++ b/tools/ui/src/lib/constants/settings-keys.ts
@@ -66,5 +66,6 @@ export const SETTINGS_KEYS = {
 	EXCLUDE_REASONING_FROM_CONTEXT: 'excludeReasoningFromContext',
 	SHOW_RAW_OUTPUT_SWITCH: 'showRawOutputSwitch',
 	// PY_INTERPRETER_ENABLED: 'pyInterpreterEnabled',
-	CUSTOM: 'custom'
+	CUSTOM_JSON: 'customJson',
+	CUSTOM_CSS: 'customCss'
 } as const;
diff --git a/tools/ui/src/lib/constants/settings-registry.ts b/tools/ui/src/lib/constants/settings-registry.ts
index efef18fdeb6..20ac33c85f8 100644
--- a/tools/ui/src/lib/constants/settings-registry.ts
+++ b/tools/ui/src/lib/constants/settings-registry.ts
@@ -659,12 +659,24 @@ const SETTINGS_REGISTRY: Record<string, SettingsSectionEntry> = {
 				}
 			},
 			{
-				key: SETTINGS_KEYS.CUSTOM,
+				key: SETTINGS_KEYS.CUSTOM_JSON,
 				label: 'Custom JSON',
 				help: 'Custom JSON parameters to send to the API. Must be valid JSON format.',
 				defaultValue: '',
 				type: SettingsFieldType.TEXTAREA,
 				section: SETTINGS_SECTION_SLUGS.DEVELOPER
+			},
+			{
+				key: SETTINGS_KEYS.CUSTOM_CSS,
+				label: 'Custom CSS',
+				help: 'CSS injected into the page at runtime. Set it here, or ship it server side via the --ui-config customCss field.',
+				defaultValue: '',
+				type: SettingsFieldType.TEXTAREA,
+				section: SETTINGS_SECTION_SLUGS.DEVELOPER,
+				sync: {
+					serverKey: SETTINGS_KEYS.CUSTOM_CSS,
+					paramType: SyncableParameterType.STRING
+				}
 			}
 		]
 	},
diff --git a/tools/ui/src/lib/services/migration.service.ts b/tools/ui/src/lib/services/migration.service.ts
index 35d47070acf..20dfa9b19fa 100644
--- a/tools/ui/src/lib/services/migration.service.ts
+++ b/tools/ui/src/lib/services/migration.service.ts
@@ -470,11 +470,36 @@ const themeMigration: Migration = {
 
 // Migration Registry & Runner
 
+const CUSTOM_JSON_MIGRATION_ID = 'custom-json-key-v1';
+
+const customJsonKeyMigration: Migration = {
+	id: CUSTOM_JSON_MIGRATION_ID,
+	description: 'Copy legacy custom config key to customJson (non-destructive)',
+
+	async run(): Promise<void> {
+		const configRaw = localStorage.getItem(CONFIG_LOCALSTORAGE_KEY);
+		if (configRaw === null) return;
+
+		const config = JSON.parse(configRaw);
+
+		if (!('custom' in config)) return;
+		if (SETTINGS_KEYS.CUSTOM_JSON in config) return;
+
+		config[SETTINGS_KEYS.CUSTOM_JSON] = config.custom;
+		localStorage.setItem(CONFIG_LOCALSTORAGE_KEY, JSON.stringify(config));
+
+		// Non-destructive: keep the legacy custom key for downgrade compatibility
+		if (import.meta.env.DEV && import.meta.env.VITE_DEBUG)
+			console.log(`[Migration] Custom JSON: copied custom to customJson (preserved old key)`);
+	}
+};
+
 const migrations: Migration[] = [
 	localStorageMigration,
 	idxdbMigration,
 	legacyMessageMigration,
-	themeMigration
+	themeMigration,
+	customJsonKeyMigration
 ];
 
 export const MigrationService = {
diff --git a/tools/ui/src/lib/stores/chat.svelte.ts b/tools/ui/src/lib/stores/chat.svelte.ts
index 5b264482602..f2f13f25dc1 100644
--- a/tools/ui/src/lib/stores/chat.svelte.ts
+++ b/tools/ui/src/lib/stores/chat.svelte.ts
@@ -1869,7 +1869,7 @@ class ChatStore {
 
 		apiOptions.backend_sampling = currentConfig.backend_sampling;
 
-		if (currentConfig.custom) apiOptions.custom = currentConfig.custom;
+		if (currentConfig.customJson) apiOptions.custom = currentConfig.customJson;
 
 		return apiOptions;
 	}
diff --git a/tools/ui/src/lib/stores/tools.svelte.ts b/tools/ui/src/lib/stores/tools.svelte.ts
index 5404a7a46fe..3ac44aedf70 100644
--- a/tools/ui/src/lib/stores/tools.svelte.ts
+++ b/tools/ui/src/lib/stores/tools.svelte.ts
@@ -57,7 +57,7 @@ class ToolsStore {
 	}
 
 	get customTools(): OpenAIToolDefinition[] {
-		const raw = config().custom;
+		const raw = config().customJson;
 		if (!raw || typeof raw !== 'string') return [];
 
 		try {
diff --git a/tools/ui/src/lib/types/settings.d.ts b/tools/ui/src/lib/types/settings.d.ts
index 65096db3449..03818091a1d 100644
--- a/tools/ui/src/lib/types/settings.d.ts
+++ b/tools/ui/src/lib/types/settings.d.ts
@@ -90,8 +90,8 @@ export interface SettingsChatServiceOptions {
 	// Sampler configuration
 	samplers?: string | string[];
 	backend_sampling?: boolean;
-	// Custom parameters
-	custom?: string;
+	// Custom JSON parameters
+	customJson?: string;
 	timings_per_token?: boolean;
 	// Continuation control (vLLM compat), opt in to the explicit continue final message flag
 	continueFinalMessage?: boolean;
diff --git a/tools/ui/src/routes/+layout.svelte b/tools/ui/src/routes/+layout.svelte
index 2f1f5249722..be474109ad9 100644
--- a/tools/ui/src/routes/+layout.svelte
+++ b/tools/ui/src/routes/+layout.svelte
@@ -169,6 +169,14 @@
 		}
 	});
 
+	// Inject custom CSS at runtime through an action on the head style node
+	// textContent keeps the value as text, never parsed as HTML
+	function customCss(node: HTMLStyleElement) {
+		$effect(() => {
+			node.textContent = (config().customCss as string | undefined) ?? '';
+		});
+	}
+
 	// Fetch router models when in router mode (for status and modalities)
 	// Wait for models to be loaded first, run only once
 	let routerModelsFetched = false;
@@ -227,6 +235,12 @@
 	});
 </script>
 
+<svelte:head>
+	{#if config().customCss}
+		<style use:customCss></style>
+	{/if}
+</svelte:head>
+
 <Tooltip.Provider delayDuration={TOOLTIP_DELAY_DURATION}>
 	<ModeWatcher />
 	<Toaster richColors />