There are two ways to open exesting document:
using menu File->Open... choice, or using
button at the toolbar.
You can open a CSS or HTML document.
If you open a CSS
document, you can edit it, attach to any HTML documents,
or insert into HTML documents.
If you open a HTML document
, data, placed in the <style> tag of that HTML document,
will be provided for editing.
Note! you can't attach or insert imported CSS into other HTML files.
You can use Tool -> Export CSS into a file... menu choice for exporting CSS from HTML file into separate CSS file.