There are two ways to open exesting document:
using menu File->Open, or using
button in the tool
bar.
You can open a CSS or HTML document.
If you will open a CSS document , you can to edit it, to attach to any
HTML document or to insert into HTML documents.
If you will open a HTML document
, data, placed in <style> tag
in that HTML document, will be parsed and you can edit it. But you will
can't to attach or insert it into other HTML file. But if you need, you can
use Tool -> Export CSS into a file...