Currently, it is not possible to get into the source code and write all of your own CSS directly. Our hope is actually that as we continue to improve Webflow, that it will almost always be faster and easier to style everything from our Webflow UI than to write it all by hand.
I'd love to hear a little bit more about why you would like to be able to write your own CSS in Webflow in addition to using the UI, though. Are there certain CSS properties missing in Webflow that you wanted to use? Are some of our tools to confusing or cumbersome to use? Or is it more of a comfort/flexibility thing?
I'd also love to hear more about how you'd like to use Webflow as a "CMS service of sorts." Would you like your clients to be able to actually edit the code on their own, or just content (i.e. text, pictures, etc.)? And would you like them to just be able to edit their own content on more "static" websites -- or would you like to give them the ability to manage content on my dynamic / data-driven websites as well, like blogs, e-commerce stores, etc.?
Thanks for the feedback, and I look forward to hearing more!