First, you’ll need to create a folder named “themes” in the root directory.

Next, Create a “demo” folder  in “themes” and create JS & CSS sub folders in “themes”

Now Add the theme related JS and CSS files in JS and CSS folder.

Create a folder named “web” and add a new Asset File which holds all js and css files.

You can configure DemoThemeAsset.php in the following way:

<?php
namespace app\themes\demo\web;
use yii\web\AssetBundle;
/**
* AssetBundle
*
* @since 0.1
*/
class DemoThemeAsset extends AssetBundle {
    public $sourcePath = ‘@app/themes/demo’;
    public $css = [‘css/demo.css’];
    public $js = [‘js/demo.js’, ];
    public $depends = [‘yii\web\JqueryAsset’, ‘yii\bootstrap\BootstrapAsset’, ‘yii\bootstrap\BootstrapPluginAsset’, ‘yii\jui\JuiAsset’, ];
    public $cssOptions = [‘position’ => \yii\web\View:
        OS_BEGIN];
        /**
         * @inheritdoc
         */
        public function init() {
            parent::init();
        }
    }
    

You’ll need to create a header.php, footer.php and main.php files in themes/demo/views/layouts folder.

Add the following details in header.php file.

<?php
clinicThemeAsset::register($this);
//Add HTML to display in the header

Add the following details in main.php file.

<?php /* @var $this \yii\web\View */ /* @var $content string */ use ktree\app\helpers\Url; use app\themes\demo\web\clinicThemeAsset; use yii\widgets\Breadcrumbs; use yii\helpers\Json; clinicThemeAsset::register($this); ?>
<?php $this->beginContent(‘@app/views/layouts/base.php’); ?>
<?=$ this->render(‘@app/views/layouts/header.php’) ?>
   <!– Breadcrumbs –>
   <div class="breadcrumbs">
      <div class="container">
         <div class="row">
            <div class="col-xs-12">
               <?=B readcrumbs::widget( [ ‘links’=> isset($this->params[‘breadcrumbs’]) ? $this->params[‘breadcrumbs’] : [], ‘options’ => [‘class’ => ‘breadcrumb’], ] ) ?>
            </div>
         </div>
      </div>
   </div>
   <!– Breadcrumbs End –>
   <div class="container">
      <?=$ content; ?>
   </div>
   <?=$ this->render(‘@app/views/layouts/footer.php’) ?>
      <?php $this->endContent(); ?>

Add below array in config/main.php components section

‘view’ => [
‘theme’ => [
‘basePath’ => ‘@app/themes/demo’,
‘baseUrl’ => ‘@app/themes/demo’,
‘pathMap’ => [
‘@app/views’ => ‘@app/themes/demo/views’,
],
],
],