Bootstrap 4

Install

composer require --prefer-dist yiisoft/yii2-bootstrap4

In composer.json

...    
    "minimum-stability": "dev",
    "require": {
        ...
        "yiisoft/yii2-bootstrap4": "^1.0@dev"
...        

in site.css (optional)

// Added by MB

.nav {
    font-size: 2.4em;
}

.nav li {
    padding-right:0px;
    font-size: 1.4em;
}


ul.nav a:hover {
    color:#a94442;
    background-color: #E0E0E0;
}

h1 {
    color:#424ca9;
    font-style: italic;
}