EasyBlogs doesn't properly process Joomla content bots. You'll need to disable "Content - Community Builder Author" within Extensions > Plug-in Manager.

When I add or change CB fields some core joomla userfiles are reset, ex. Prefered frontend language, prefered editor, prefered backend language.

No idea what you mean by this. You don't see Joomlas profile or fields.

CB Connect was installed, but FB icon didn't show until I fixed it in the css file. Strange that this is not working as others may encounter the same problem.

It is not an "install and done" plugin. It requires configuration. See CB > Plugin Management > CB Connect > Help.

Where can I find the css files to style the CB login module?

No where, we don't style the module. It uses Joomla classes so it can be styled by your Joomla template. Inspect with Firebug for Firefox or Chrome to see its classes and IDs for styling in your Joomla template.

Please carefully read your CB documentation, which is available for download within Advanced downloads section. It will walk you through basic CB usage.