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It is possible to change definitions such as app descriptors, dialogs, field types, message views, templates, media editors and renderer definitions. Any definition bound to a  

Decorated definitions can originate from any source, including the JCR, YAML files or  even executable code like Blossom. However, decorators themselves can only be defined via YAML for the time being. 

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*) A Magnolia Maven module is a Maven packaged module which contains a Magnolia Module descriptor.
**) The Module descriptor defines Module dependencies. If module-a depends on module-b, module-b is loaded before module-a. The module dependencies of all Magnolia Maven modules together define a distinct order. 

Decorating for merging or overriding

Have a look at Changing the cssFiles property in the mtk page template basic for examples of both merging and overriding.

Definition decorator file location

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my-maven-module/
└── src
    └── main
        └── resources
            └── my-module
                └── decorations
                    ├── dam-app
                    │   └── apps
                    │       └── assets.subApps.browser.contentConnector.yaml
                    ├── mtk
                    │   └── templates
                    │       └── pages
                    │           ├── basic.cssFiles.yaml
                    │           └── basic.yaml
                    └── pages
                        └── apps
                            └── pages.yaml          

$magnolia.resources.dir
└── my-module
    └── decorations
        ├── dam-app
        │   └── apps
        │       └── assets.subApps.browser.contentConnector.yaml
        ├── mtk
        │   └── templates
        │       └── pages
        │           ├── basic.cssFiles.yaml
        │           └── basic.yaml
        └── pages
            └── apps
                └── pages.yaml

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Change the title and the icon of an app

label: My pages ...
icon: icon-user-me

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Change the root path of the content connector of the assets app

rootPath: /cars/007-cars

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(Screenshot was taken from the definition-app which shows definition items read from the corresponding registry.) 

Add a css file to the mtk page template basic

cssFiles:
  chmStyles:
    link: /.resources/my-module/css/chm-style.css

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Changing the cssFiles property in the mtk page template basic

chmStyles:
  link: /.resources/my-module/css/chm-style.css

Overriding properties and changing subitems

To understand the above statement, let's have a look at an example of a (simplified) template definition:

templateScript: /mtk/templates/pages/basic.ftl
dialog: mtk:pages/basic
renderType: freemarker
class: info.magnolia.module.templatingsite.definitiontemplates.PageTemplateDefinition
cssFiles:
  normalize:
    link: /.resources/mtk/webresources/css/normalize-3.0.3.css
  main:
    link: /.resources/mtk/webresources/css/html5boilerplate-main-5.3.0.css
  video:
    link: /.resources/mtk/webresources/css/video.css

Here you can completely override the properties templateScript, dialog, renderType, class and link, but you cannot completely override the cssFiles subItem. For subitems you can only can add more properties and subItems.

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The original template defines three CSS files.

a) Adding an additional CSS file - merging

cssFiles:
  minimum:
    link: /.resources/my-module/webresources/css/minimum.css

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b) Overriding the property completely - using !override

normalizecssFiles: !override
  minimum:
    link: ""/.resources/my-module/webresources/css/minimum.css

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