do you know how can i set fs clock as screensasver?

Sure, just go into themes and scroll to the theme that you're currenly using. Hit options and then edit. The 2nd item there is Power saver. Just scroll to that and select the type of screensaver you want.

yes i know but in themes it has not the choice fs clock. It has only date and time, calendar, text and slide screensaver

I've never tried the software so I can't really say. Why not just leave the app running? Try contacting the author (Smartphoneware) or reading any guide or manual that came with the s/w.

ok thanks