Theme instilation problem

Ok heres my problem.I went to change my theme to a vista theme i found on hitskin.However when i installed it,the whole site was green exept for colors of peoples usernames.So i installed my regular phpbb blue skin back.Then it was normal for like 10 minutes then it went back to the vista theme only this time it
was normal vista not green.This time the nav bar was messed up so i tried to change it back again, and it worked.I had my blue skin back.I went to change the phpbb logo to my site logo again then somehow i got the whole vista the back again.Know every time i change my theme back to the phpbb blue it turns into the vista theme within 10 minutes.My members also tell me that its still vista theme for them when it is blue for me.Please help!!

PS:I hope this is in the right section