# Theming ¶

Theming is a way to replace a set of views with another without the need of touching the original view rendering code. You can use theming to systematically change the look and feel of an application.

To use theming, you should configure the theme property of the `view` application component. The property configures a yii\base\Theme object which governs how view files are being replaced. You should mainly specify the following properties of yii\base\Theme:

- yii\base\Theme::$basePath: specifies the base directory that contains the themed resources (CSS, JS, images, etc.)
- yii\base\Theme::$baseUrl: specifies the base URL of the themed resources.
- yii\base\Theme::$pathMap: specifies the replacement rules of view files. More details will be given in the following subsections.

For example, if you call `$this->render('about')` in `SiteController`, you will be rendering the view file `@app/views/site/about.php`. However, if you enable theming in the following application configuration, the view file `@app/themes/basic/site/about.php` will be rendered, instead.

    return [
        'components' => [
            'view' => [
                'theme' => [
                    'basePath' => '@app/themes/basic',
                    'baseUrl' => '@web/themes/basic',
                    'pathMap' => [
                        '@app/views' => '@app/themes/basic',
                    ],
                ],
            ],
        ],
    ];

> Info: Path aliases are supported by themes. When doing view replacement, path aliases will be turned into the actual file paths or URLs.

You can access the yii\base\Theme object through the yii\base\View::$theme property. For example, in a view file, you can write the following code because `$this` refers to the view object:

    $theme = $this->theme;

    // returns: $theme->baseUrl . '/img/logo.gif'
    $url = $theme->getUrl('img/logo.gif');

    // returns: $theme->basePath . '/img/logo.gif'
    $file = $theme->getPath('img/logo.gif');

The yii\base\Theme::$pathMap property governs how view files should be replaced. It takes an array of key-value pairs, where the keys are the original view paths to be replaced and the values are the corresponding themed view paths. The replacement is based on partial match: if a view path starts with any key in the pathMap array, that matching part will be replaced with the corresponding array value. Using the above configuration example, because `@app/views/site/about.php` partially matches the key `@app/views`, it will be replaced as `@app/themes/basic/site/about.php`.

## Theming Modules ¶

In order to theme modules, yii\base\Theme::$pathMap can be configured like the following:

    'pathMap' => [
        '@app/views' => '@app/themes/basic',
        '@app/modules' => '@app/themes/basic/modules', // <-- !!!
    ],

It will allow you to theme `@app/modules/blog/views/comment/index.php` into `@app/themes/basic/modules/blog/views/comment/index.php`.

## Theming Widgets ¶

In order to theme widgets, you can configure yii\base\Theme::$pathMap in the following way:

    'pathMap' => [
        '@app/views' => '@app/themes/basic',
        '@app/widgets' => '@app/themes/basic/widgets', // <-- !!!
    ],

This will allow you to theme `@app/widgets/currency/views/index.php` into `@app/themes/basic/widgets/currency/views/index.php`.

## Theme Inheritance ¶

Sometimes you may want to define a basic theme which contains a basic look and feel of the application, and then based on the current holiday, you may want to vary the look and feel slightly. You can achieve this goal using theme inheritance which is done by mapping a single view path to multiple targets. For example,

    'pathMap' => [
        '@app/views' => [
            '@app/themes/christmas',
            '@app/themes/basic',
        ],
    ]

In this case, the view `@app/views/site/index.php` would be themed as either `@app/themes/christmas/site/index.php` or `@app/themes/basic/site/index.php`, depending on which themed file exists. If both themed files exist, the first one will take precedence. In practice, you would keep most themed view files in `@app/themes/basic` and customize some of them in `@app/themes/christmas`.