As a lightweight editor that syntax highlights and lets you edit css and php i would recommend notepad++. It will syntax highlight a lot of languages (html/php/css are among them). But there are many others.
You should download and use CKEditor which is the editor that MailChimp and many other major companies use, it is open source and combined with the syntax highlighter plugin it's the perfect tool for integrating into your own custom web application to edit CSS and other files
Updating my answer
Ckeditor has syntax highlighting built in see the demo here