In prosilver style of Arabic language which uses bidi.css as it is a right to left language, the background "the arrow" of "back to top link" using the class "top2" is in wrong position, instead of being on the right side of the word it is in the left side.
I solved the problem by adding this to the bidi.css file after .rtl a.top2 { :

background-position:100% 50%;