add client styling chapter

framework/client/styling.rst

@@ -0,0 +1,45 @@
+Styling
+==============
+If tocco apps, for example tocco-login or entity-browser, are integrated in 3rd party applications or content management systems, there 
+is the need to customize the apps in terms of the visual appearance to match the rest of the application.
+
+Custom Theme
+-------------
+Every app supports the input property *customTheme*, which is an object with a defined structure.
+Inside the tocco-theme package there is a default-theme defined. Whenever no *customTheme* is provided to an app, the default theme will be used
+as a fallback. If a *customTheme* is provided with only a few properties defined (e.g. only the colors), the *customTheme* will be merged with the default theme.
+
+The `Default Theme`_ can be found here.
+
+.. _Default Theme: https://github.com/tocco/tocco-client/blob/master/packages/tocco-theme/src/ToccoTheme/defaultTheme.js
+
+
+This is a valid example of a *customTheme* only changing the primary color and the used font:
+
+.. code-block:: json           
+
+  {
+      "colors": {
+          "primary": "#36ff00"
+      },
+      "fontFamily": {
+          "regular": "'Helvetica Neue', Helvetica, Arial, sans-serif"
+      }
+  }
+
+
+
+Nice2 Integration
+------------------
+Apps used in Nice2 are only using the default theme at the moment. But apps used as widgets in the cms support a standardized way of adding a customTheme.
+
+The following file should define a global variable named ``customerTheme`` containing a valid *customTheme* (or parts of it): 
+``customer/{CUSTOMER}/share/cms/web/js/tocco-client-theme.js``
+
+In the `page_header.ftl` this file is loaded and the `reactRegistry.js` will check, if the global variable is defined and passes it to the tocco app if yes.
+
+Of course this only gives control over a handful of parameters. If more customizing is wanted by the customer, there is still the way to add css on top of everything. 
+But such styling will never be supported and might break with an update of a client packages.
+
+
+