Magento 2 : How to add custom css for adminhtml?
1 Create a module for example Vendor/Module
2 Create a new file view/adminhtml/layout/default.xml in the module
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="admin-1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<css src="Vendor_Module::css/test.css"/>
</head>
</page>
3) Add .css file in web folder as below path:
view/adminhtml/web/css/test.css
Run below commands and check.
php bin/magento cache:clean
php bin/magento setup:static-content:deploy
Magento 2 uses less file so you can also use less type as follows below step.
Create new file at Vendor/Module/view/adminhtml/web/css/source/_module.less
after that run below command from root directory and check
rm -rf var/view_preprocessed/
rm -rf pub/static/adminhtml
php bin/magento setup:static-content:deploy