[BUG] Minor style issues and wrong links for the plain option

[lguariento]

Generating an eXist-db application with the latest 5.1.0 version and choosing the plain template, the navigation has minor css issues:

A working code snippet for the navigation in Bootstrap 5.1.3 is:

        <nav class="navbar navbar-expand-lg navbar-dark bg-dark">
            <div class="container-fluid">
                <a class="navbar-brand" href="./index.html">Navbar</a>
                <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNavDarkDropdown" aria-controls="navbarNavDarkDropdown" aria-expanded="false" aria-label="Toggle navigation">
                    <span class="navbar-toggler-icon"></span>
                </button>
                <div class="collapse navbar-collapse" id="navbarNavDarkDropdown">
                    <ul class="navbar-nav">
                        <li class="nav-item dropdown">
                            <a class="nav-link dropdown-toggle" href="#" id="navbarDarkDropdownMenuLink" role="button" data-bs-toggle="dropdown" aria-expanded="false">
                                Dropdown
                            </a>
                            <ul class="dropdown-menu dropdown-menu-dark" aria-labelledby="navbarDarkDropdownMenuLink">
                                <li><a class="dropdown-item" href="#">Action</a></li>
                                <li><a class="dropdown-item" href="#">Another action</a></li>
                                <li><a class="dropdown-item" href="#">Something else here</a></li>
                            </ul>
                        </li>
                    </ul>
                </div>
            </div>
        </nav>

Also, the link for Bootstrap in the start page:

The page template uses the Bootstrap CSS library for the page layout.

is a 404.