Whit NPP closed select the folder in Explorer than drag and drop it on the NPP shortcut (e.g. the one your Desktop), it should open all files in the “old way”
Select Style Configurator... in the Settings menu, select e.g. HTML in the Language list, the Style list will show the descriptions of the items that are colored. Select one of these descriptions to show the specific color and font style for that item.
Notepad++ does not indicate errors in source code natively.
Maybe there’s a dedicated plugin with this functionality but I don’t know of any.
You could Google for a HTML and CSS Validator.