Right click > inspect > on refresh indicator right click and select empty cache and hard reload. Maybe the css is not properly linked here, try:

<link rel="stylesheet" type="text/css" th:href="@{/style.css}"/> or <link rel="stylesheet" type="text/css" href="/style.css"/>

maybe try moving css to another location.

and what about the configuration?

@Bean
public SpringResourceTemplateResolver templateResolver() {
    SpringResourceTemplateResolver templateResolver = new SpringResourceTemplateResolver();
    templateResolver.setPrefix("classpath:/templates/");
    templateResolver.setSuffix(".html");
    templateResolver.setTemplateMode("HTML");
    return templateResolver;
}