However you strip the DRM of your books, if you wish to maintain the original CSS of your kindle books (without converting them which modifies the css somewhat, but not breaking it) there is a plugin for calibre called "KindleUnpack - The Plugin" which essentially changes the container of the ebook keeping the contents of your ebook intact as they appear in a azw format.
To add it open calibre, open preferences/settings ->plugins ->get new plugins - search "kindleunpack" and follow instructions to install it from there.
Here is a link to the plugins page on the forum