Feature: #100232 - Load additional stylesheets in TYPO3 backend

See forge#100232

Description

It is now possible to load additional CSS files for the TYPO3 backend interface via regular $TYPO3_CONF_VARS settings in a settings.php file of a project (previously known as LocalConfiguration.php) file or in an extension's ext_localconf.php.

Previously this was done via the outdated $TBE_STYLES global array which has been deprecated.

Impact

By defining a specific stylesheet, a single CSS file or all CSS files of a folder, extension authors can now modify the styling via:

EXT:my_extension/ext_localconf.php 
$GLOBALS['TYPO3_CONF_VARS']['BE']['stylesheets'][my_extension]
    = 'EXT:myextension/Resources/Public/Css/myfile.css';

$GLOBALS['TYPO3_CONF_VARS']['BE']['stylesheets'][my_extension]
    = 'EXT:myextension/Resources/Public/Css/';
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in their extension's ext_localconf.php file.

Site administrators can handle this in their settings.php or additional.php file.