这是因为在Angular 12中，'styleext”已被删除，而被'style”替换。 要解决这个问题，需要在项目的.angular-cli.json或angular.json文件中，'styleext”属性改为'style”。 示例代码如下：

{ "$schema": "./node_modules/@angular/cli/lib/config/schema.json", "version": 1, "newProjectRoot": "projects", "projects": { "my-project-name": { "root": "", "sourceRoot": "src", "projectType": "application", "prefix": "app", ... "schematics": {}, "architect": { "build": { "builder": "@angular-devkit/build-angular:browser", "options": { "outputPath": "dist/my-project-name", "index": "src/index.html", "main": "src/main.ts", "tsConfig": "src/tsconfig.app.json", "polyfills": "src/polyfills.ts", "assets": [ "src/favicon.ico", "src/assets" ], "styles": [ "src/styles.css" ], "scripts": [] }, ... }, "serve": { "builder": "@angular-devkit/build-angular:dev-server", "options": { "browserTarget": "my-project-name:build" }, "configurations": { "production": { "browserTarget": "my-project-name:build:production" } } }, "extract-i18n": { "builder": "@angular-devkit/build-angular:extract-i18n", "options": { "browserTarget": "my-project-name:build" } }, "test": { "builder": "@angular-devkit/build-angular:karma", "options": { "main": "src/test.ts", "polyfills": "src/polyfills.ts", "tsConfig": "src/tsconfig.spec.json", "karmaConfig": "src/karma.conf.js", "assets": [ "src/favicon.ico", "src/assets