I am using multiple editors and swtich according to the requirements. Sounds strange but yes I do.
Theme development require more work on stylesheet and Notepad++ has some limited features for writing stylesheet in fast even with some plugins. It doesn't mean I dont like Notepad++ in fact I love that. But I mostly use it for php and html more than stylesheet. In fact I have reduce the use of NPP since I found two great editors.
Sublime Text 2
phpdesigner8 has great capability to work on PHP projects. Its wonderful and allows jump to the core function quickly than ever I seen. There are many other featuers too. But drawback: It's a paid and not free
Sublime Text 2 has great capability to work on HTML and CSS specially. It is damn fast in writing styelsheet and even supports css3 coding which makes production much faster and errorless. Good thing about it is completely free to use. Paid version also available.
For above both editors has unlimited features to discribe here better you should go to them website and check it out.