在 Yii2 Starter Kit 的后台应用中，新建一套主题的流程

1、新建目录：\backend\views\themes\default，指定包含主题资源(CSS, JS, images, 等等)的基准目录，如图1

2、复制目录：\backend\views\ 下的所有文件(除去themes)，如图2

3、粘贴至目录：\backend\views\themes\default\ 下，已经复制成功，如图3

4、配置 view 应用组件的 [[yii\base\View::theme|theme]] 属性，如图4

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

5、配置 Gii 模板文件，其主题为：default，如图5

if (YII_ENV_DEV) {
    $config['modules']['gii'] = [
        'class' => yii\gii\Module::class,
        'generators' => [
            'crud' => [
                'class' => yii\gii\generators\crud\Generator::class,
                'templates' => [
                    'yii2-starter-kit' => Yii::getAlias('@backend/views/themes/default/_gii/templates')
                ],
                'template' => 'yii2-starter-kit',
                'messageCategory' => 'backend'
            ]
        ]
    ];
}

6、主题化模块，如：i18n ，新建目录：\backend\modules\i18n\views\themes\default，如图6

7、复制目录：\backend\modules\i18n\views\ 下的所有文件(除去themes)，如图7

8、粘贴至目录：\backend\modules\i18n\views\themes\default\ 下，已经复制成功，如图8

9、配置 view 应用组件的 [[yii\base\View::theme|theme]] 属性，如图9

        'view' => [
            'theme' => [
                'basePath' => '@app/views/themes/default',
                'baseUrl' => '@web/views/themes/default',
                'pathMap' => [
                    '@app/views' => '@app/views/themes/default',
                    '@app/modules/i18n/views' => '@app/modules/i18n/views/themes/default',
                ],
            ],
        ],

10、通过 [[yii\base\View::theme]] 属性访问 [[yii\base\Theme]] 对象。编辑：\backend\views\themes\default\sign-in\login.php，如图10

$theme = $this->theme;
print_r($theme);
exit;

11、打印出的对象，如图11

yii\base\Theme Object
(
[pathMap] => Array
(
[@app/views] => @app/views/themes/default
[@app/modules/i18n/views] => @app/modules/i18n/views/themes/default
)

[_baseUrl:yii\base\Theme:private] => /views/themes/default
[_basePath:yii\base\Theme:private] => E:\wwwroot\cmcp\backend/views/themes/default
[_events:yii\base\Component:private] => Array
(
)

[_behaviors:yii\base\Component:private] =>
)

12、在 .env 中设置主题名称：default，如图12

# Themes
# ----
BACKEND_THEME = default

13、设置主题相关别名，编辑：\backend\config\bootstrap.php，定义别名，如图13

/**
 * Setting theme aliases
 */
Yii::setAlias('@themePath', '@backend/web/themes/' . env('BACKEND_THEME'));
Yii::setAlias('@themeUrl', '@backendUrl/themes/' . env('BACKEND_THEME'));

14、第5步骤的设置，调整，如图14

if (YII_ENV_DEV) {
    $config['modules']['gii'] = [
        'class' => yii\gii\Module::class,
        'generators' => [
            'crud' => [
                'class' => yii\gii\generators\crud\Generator::class,
                'templates' => [
                    'yii2-starter-kit' => Yii::getAlias('@backend/views/themes/' . env('BACKEND_THEME') . '/_gii/templates')
                ],
                'template' => 'yii2-starter-kit',
                'messageCategory' => 'backend'
            ]
        ]
    ];
}

15、第9步骤的设置，调整，如图15

        'view' => [
            'theme' => [
                'basePath' => '@app/views/themes/' . env('BACKEND_THEME'),
                'baseUrl' => '@web/views/themes/' . env('BACKEND_THEME'),
                'pathMap' => [
                    '@app/views' => '@app/views/themes/' . env('BACKEND_THEME'),
                    '@app/modules/i18n/views' => '@app/modules/i18n/views/themes/' . env('BACKEND_THEME'),
                ],
            ],
        ],

16、新建目录：\backend\web\themes\default，指定包含主题资源(CSS, JS, images, 等等)的基准目录，如图16

17、复制目录：\backend\web\ 下的目录(css、img、js)，如图17

18、粘贴至目录：\backend\web\themes\default\ 下，已经复制成功，如图18

19、基于主题重新定义资源包，编辑：\backend\assets\BackendAsset.php，如图19

    public $basePath = '@themePath';
    public $baseUrl = '@themeUrl';

20、查看登录页面的源代码，最大的变化在于包含的CSS/JS文件，其路径为绝对路径，如图20