php - Correct way of app/design and app/code usage Magento 2 Adminhtml -

i wish understand 2 things:

1. extend magento 2 admin css ( or correct way doing less?) copy css/less original back-end
2. override in back-end example : sales > orders > called file vendor/magento/module-backend/view/adminhtml/templates/pageactions.phtml

i have followed http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/admin_theme_create.html & http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/admin_theme_apply.html

checked different stackoverflow posts like

magento 2.0 override admin css (style.css)

but i'm @ loss @ moment understanding correctly.

i wish override css magento login example. when use app/code, , when use app/design , reasons? in example i'm trying both , i'm not sure place reason. posts refer overriding through app/code, app/design css change works too, correct?

i succesful in creating custom page backend, through module activate custom adminhtml template through di.xml, , show own logo on admin login works, have custom css. won't bother parts working i'll leave index.php, menu.xml, routes.xml, exampleadminnewpage_helloworld_index , files in templates out of because works fine.

app/code file structure

        app/code/vendor            └── exampleadminnewpage             ├── composer.json             ├── controller             │   └── adminhtml             │       └── helloworld             │           └── index.php             ├── etc             │   ├── adminhtml             │   │   ├── menu.xml             │   │   └── routes.xml             │   ├── di.xml             │   └── module.xml             ├── registration.php             └── view                 └── adminhtml                     ├── layout                     │   └── exampleadminnewpage_helloworld_index.xml                     └── templates                         ├── helloworld.phtml                         ├── intro.css                         └── js.phtml 

app/code/vendor/exampleadminnewpage/etc/di.xml

        <config xmlns:xsi="http://www.w3.org/2001/xmlschema-instance"                 xsi:nonamespaceschemalocation="urn:magento:framework:objectmanager/etc/config.xsd">             <type name="magento\theme\model\view\design">                 <arguments>                     <argument name="themes" xsi:type="array">                         <item name="adminhtml" xsi:type="string">vendor/test_admin</item>                     </argument>                 </arguments>             </type>         </config> 

app/code/vendor/exampleadminnewpage/registration.php

    <?php     \magento\framework\component\componentregistrar::register(         \magento\framework\component\componentregistrar::module,         'vendor_exampleadminnewpage',         __dir__     ); 

app/code/vendor/exampleadminnewpage/etc/module.xml

        <?xml version="1.0"?>         <config xmlns:xsi="http://www.w3.org/2001/xmlschema-instance" xsi:nonamespaceschemalocation="urn:magento:framework:module/etc/module.xsd">             <module name="vendor_exampleadminnewpage" setup_version="1.0.0">                 <sequence>                     <module name="magento_backend"/>                 </sequence>             </module>         </config> 

app/design file structure

     app/design/adminhtml/vendor         └── test_admin             ├── magento_sales             │   └── templates             │       ├── items             │       │   └── column             │       │       └── name.phtml             │       └── pageactions.phtml             ├── registration.php             ├── theme.xml             └── web                 ├── css                 │   └── styles.css                 ├── images                 │   └── magento-logo.svg > own logo , shows @ admin login!                 └── js                     └── requirejs-config.js 

the following echo doesn't work, had expected if went sales>orders, i'd see echo? app/design/adminhtml/vendor/test_admin/magento_sales/templates/items/column/name.phtml

    <?php echo 'test'; ?> 

same goes echo, expected see echo @ sales>orders
app/design/adminhtml/vendor/test_admin/magento_sales/templates/pageactions.phtml

    <?php echo 'testpageaction'; ?> 

the css works, supposed through way? , can find original back-end css/less?
app/design/adminhtml/vendor/test_admin/web/css/styles.css

    span {         background-color: red !important;     } 

i don't know how js working, have. app/design/adminhtml/vendor/test_admin/web/js/requirejs-config.js

var config = {     map: {         '*': {             customadmin: 'vendor_test_admin/js/customadmin'         }     },     deps: ["jquery"] }; 

app/design/adminhtml/vendor/test_admin/registration.php

<?php /**  * copyright © 2016 magento. rights reserved.  * see copying.txt license details.  */ \magento\framework\component\componentregistrar::register(     \magento\framework\component\componentregistrar::theme,     'adminhtml/vendor/test_admin', // example: 'adminhtml/magento/backend'     __dir__ ); 

public_html/app/design/adminhtml/vendor/test_admin/theme.xml

<theme xmlns:xsi="http://www.w3.org/2001/xmlschema-instance" xsi:nonamespaceschemalocation="urn:magento:framework:config/etc/theme.xsd">     <title>test_admin theme</title> <!-- theme's name -->     <parent>magento/backend</parent> <!-- parent theme. example: magento/backend --> </theme> 

i hope question clear, it's bit unclear me , that's why maybe can't word correctly hope have enough.