How can I modify the Admin Theme?

Issue

Sometimes we need to modify some aspects of the admin-theme, for example, to change the look and feel of web content editions.

The admin-theme can not be modified directly, we need to create a new theme based on the admin-theme and then modify this new custom admin-theme.
Steps to create the custom-admin-theme:
1. Create a new theme following the steps described in this article Creating Themes. custom-admin-theme will be used as its name.
2. Copy the following folders from LIFERAY_SRC_7_x/modules/apps/frontend-theme/frontend-theme-admin/src to the /src folder of custom-admin-theme:
```
CSS
Templates
Images
```
3. Edit /src/WEB-INF/liferay-look-and-feel.xml and replace the xml tags included between the tag <theme>...</theme> by <control-panel-theme>true</control-panel-theme>, like this:
```
<?xml version="1.0"?>
<!DOCTYPE look-and-feel PUBLIC "-//Liferay//DTD Look and Feel 7.1.0//EN" "http://www.liferay.com/dtd/liferay-look-and-feel_7_1_0.dtd">

<look-and-feel>
	<compatibility>
		<version>
			7.1.0+
		</version>
	</compatibility>
	<theme id="custom-admin-theme" name="custom-admin-theme">
		<control-panel-theme>
			true
		</control-panel-theme>
	</theme>
</look-and-feel>
```
4. Make the modifications that you need. For example, to modify some css, edit /src/css/_custom.scss and add or modify the css rules you need.
5. Compile the custom-admin-theme and deploy it into Liferay.
6. To apply the new theme, please access Control Panel > Configuration > Instance Settings > Miscellaneous > Look and Feel > Default Control Panel Theme and select custom-admin-theme.