Spring and Gradle – adding to the project WebJars Bootstrap

If you wanted to add bootstrap UI CSS and JS files in thymeleaf then you can use following code:

<link th:href="@{/webjars/bootstrap/5.0.0/css/bootstrap.min.css}"
      rel="stylesheet" media="screen" />
<script th:src="@{/webjars/bootstrap/5.0.0/js/bootstrap.min.js}"></script>

If those styles are still not loading then refresh project and if you are using spring security then you need to add following code in your extended class of

WebSecurityConfigurerAdapter

@Override
protected void configure(HttpSecurity http) throws Exception {
    http
            .authorizeRequests()
            .antMatchers("/", "/home","/webjars/**", "/resources/**").permitAll()
            .anyRequest().authenticated()
            .and()
            .formLogin()
            .loginPage("/login")
            .permitAll()
            .and()
            .logout()
            .permitAll();
}

It will solve your issue