Is link/PR juice being passed to CSS, favicon, thumbnail files?
If yes, how to combine
Yeah- No. Only for content. Think pages and pages only.
Google designed PageRank to indicate what web pages are important. It is designed to valuate content, not ancillary resources that have no value of their own. Since it's inception, it has been expanded to include content resources such as PDF, word documents, spreadsheets and anything else that can be considered content but not extended to non-content elements.