Fixes microsoft/monaco-editor#2416

src/css/css.test.ts

@@ -577,5 +577,17 @@ testTokenization('css', [
 				{ startIndex: 8, type: 'delimiter.bracket.css' }
 			]
 		}
+	],
+
+	[
+		{
+			line: "@import 'https://example.com/test.css';",
+			tokens: [
+				{ startIndex: 0, type: 'keyword.css' },
+				{ startIndex: 7, type: '' },
+				{ startIndex: 8, type: 'string.css' },
+				{ startIndex: 38, type: 'delimiter.css' }
+			]
+		}
 	]
 ]);

src/css/css.ts

@@ -113,6 +113,7 @@ export const language = <languages.IMonarchLanguage>{
 			{ include: '@functioninvocation' },
 			{ include: '@numbers' },
 			{ include: '@name' },
+			{ include: '@strings' },
 			['([<>=\\+\\-\\*\\/\\^\\|\\~,])', 'delimiter'],
 			[',', 'delimiter']
 		],