Hello,
I had the exact same issue here.
If you have magento's core file in the vendor directory following the tutorial from the link you provided doesn't work.
For some reason it doesn't go get the files in app/design/adminhtml/Magento/backend
Follow the tutorial but instead of creating the _module.less file in
app/design/adminhtml/Magento/backend/Your_Module/web/css/source
create it in:
vendor/magento/theme-adminhtml-backend/Your_Module/web/css/source
Doing this is not recommended since you shouldn't be touching those files but I didn't find any other solution
To make sure it worked check the following file on your server:
pub/static/adminhtml/Magento/backend/en_US/css/styles.less
Where you should find a line like this:
@import '../Your_Module/css/source/_module.less';
If you can find this line it should work ok
Hope this helps
