For example, if I open a text file, notepad++ would open it in 'default' theme. But if I open a php file for instance, it would open it in 'custom_theme1'. I know notepad++ is highly extensible, so I wasn't sure if anybody had ever run across a plugin that could do it. Or could a plugin even be written to do this?
Unfortunately, at this time it appears that this functionality is not a part of Notepad++. However, it has been submitted to the developers as a feature request.