Hi Satadru,
You cannot import HTML/CSS to Webflow, you need to build your layouts in the designer.
There are some 3rd party efforts in this area, such as AI tools that are attempting to support HTML → Webflow translation, as well as a new one in process for Wordpress → Webflow migration.
You might search for those.
I’ve some notes here, I think HTFlow is the AI one ( haven’t tested it ).