Build maven - how to include the style.css ?

Vaadin 6 / 7 / 8
1

View :

@StyleSheet("styles.css")

Style :

src/main/webapp/frontend/styles.css

pom.xml:

[...]

<packaging>jar</packaging>
[...]

	<pluginRepositories>
		<pluginRepository>
			<id>vaadin-prereleases</id>
			<url>https://maven.vaadin.com/vaadin-prereleases</url>
		</pluginRepository>
	</pluginRepositories>

	<repositories>
		<repository>
			<id>vaadin-prereleases</id>
			<url>https://maven.vaadin.com/vaadin-prereleases</url>
		</repository>
		<repository>
			<id>Vaadin Directory</id>
			<url>http://maven.vaadin.com/vaadin-addons</url>
		</repository>
	</repositories>
		<dependencyManagement>
		<dependencies>
			<dependency>
				<groupId>com.vaadin</groupId>
				<artifactId>vaadin-bom</artifactId>
				<version>${vaadin.version}</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>
	[...]

	<build>
		<plugins>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.8.0</version>
				<configuration>
					<release>10</release>
				</configuration>
			</plugin>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
			<plugin>
				<artifactId>maven-surefire-plugin</artifactId>
				<version>2.22.0</version>
			</plugin>
		</plugins>
	</build>
	<profiles>
		<profile>
			<id>production</id>

			<dependencies>
				<dependency>
					<groupId>com.vaadin</groupId>
					<artifactId>flow-server-production-mode</artifactId>
				</dependency>
			</dependencies>

			<build>
				<plugins>
					<plugin>
						<groupId>com.vaadin</groupId>
						<artifactId>vaadin-maven-plugin</artifactId>
						<version>${vaadin.version}</version>
						<executions>
							<execution>
								<goals>
									<goal>copy-production-files</goal>
									<goal>package-for-production</goal>
								</goals>
							</execution>
						</executions>
					</plugin>
				</plugins>
			</build>
		</profile>
	</profiles>

Problem: the styles.css is not include in the final jar…
What is the best practice here ? How should I package my app ? I don’t find resources or example to help…