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
1<?php
2namespace app\themes\demo\web;
3use yii\web\AssetBundle;
4/**
5* AssetBundle
6*
7* @since 0.1
8*/
9class DemoThemeAsset extends AssetBundle {
10    public $sourcePath = '@app/themes/demo';
11    public $css = ['css/demo.css'];
12    public $js = ['js/demo.js', ];
13    public $depends = ['yii\web\JqueryAsset', 'yii\bootstrap\BootstrapAsset', 'yii\bootstrap\BootstrapPluginAsset', 'yii\jui\JuiAsset', ];
14    public $cssOptions = ['position' => \yii\web\View:
15        OS_BEGIN];
16        /**
17         * @inheritdoc
18         */
19        public function init() {
20            parent::init();
21        }
22    }

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
1<?php
2clinicThemeAsset::register($this);
3//Add HTML to display in the header

Add the following details in main.php file.

PHP
1<?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); ?>
2<?php $this->beginContent('@app/views/layouts/base.php'); ?>
3<?=$ this->render('@app/views/layouts/header.php') ?>
4   <!-- Breadcrumbs -->
5   <div class="breadcrumbs">
6      <div class="container">
7         <div class="row">
8            <div class="col-xs-12">
9               <?=B readcrumbs::widget( [ 'links'=> isset($this->params['breadcrumbs']) ? $this->params['breadcrumbs'] : [], 'options' => ['class' => 'breadcrumb'], ] ) ?>
10            </div>
11         </div>
12      </div>
13   </div>
14   <!-- Breadcrumbs End -->
15   <div class="container">
16      <?=$ content; ?>
17   </div>
18   <?=$ this->render('@app/views/layouts/footer.php') ?>
19      <?php $this->endContent(); ?>

Add below array in config/main.php components section

JavaScript
1'view' => [
2'theme' => [
3'basePath' => '@app/themes/demo',
4'baseUrl' => '@app/themes/demo',
5'pathMap' => [
6'@app/views' => '@app/themes/demo/views',
7],
8],
9],