Description

This is similar issue as what was being reported in #23619
But this time it's for the Magento/backend theme.
The fact that there are errors in the less code might indicate that certain things aren't getting outputted in css properly. But without deeper research I can't confirm this yet.

Preconditions (*)

1. Magento 2.3-develop branch (or any older released version)
2. NodeJs (I'm using v8.16.0)
3. PHP 7.2

Steps to reproduce (*)

1. Setup a vanilla Magento installation
2. Run npm install less@3.9.0
3. Run rm -R var/view_preprocessed/* pub/static/*; bin/magento setup:static-content:deploy -f --theme=Magento/backend en_US
4. Run node_modules/.bin/lessc -l var/view_preprocessed/pub/static/adminhtml/Magento/backend/en_US/css/styles-old.less

Expected result (*)

1. Linting of the styles-old.less file shouldn't show any errors

Actual result (*)

1. Linting of the styles-old.less file shows the following errors:

extend ' .data-table tbody tr:nth-child(odd) td' has no matches extend ' .grid-actions .export .label' has no matches extend ' .grid-actions .export .action-' has no matches extend ' .col-570' has no matches