yii2/docs/guide/theming.md

Theming

A theme is a directory of view and layout files. Each file of the theme overrides corresponding file of an application when rendered. A single application may use multiple themes and each may provide totally different experience. At any time only one theme can be active.

Note: Themes usually do not meant to be redistributed since views are too application specific. If you want to redistribute customized look and feel consider CSS and JavaScript files in form of asset bundles instead.

Configuring current theme

Theme configuration is specified via view component of the application. So in order to set it up the following should be in your application config file:

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

In the above pathMap defines where to look for view files while baseUrl defines base URL for resources referenced from these files. In our case pathMap is ['@app/views' => '@app/themes/basic'] so the themed version for a view file @app/views/site/index.php will be @app/themes/basic/site/index.php.

Using multiple paths

It is possible to map a single path to multiple paths. For example,

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

In this case, the view will be searched in @app/themes/christmas/site/index.php then if it's not found it will check @app/themes/basic/site/index.php. If there's no view there as well application view will be used.

This ability is especially useful if you want to temporary or conditionally override some views.