Keep current theme

Hi…..

When i editing the css file of themes that i’m currently using, then i click a menu or a post in my own blog, hence the themes that i’m currently using will sudden changes to the default/kubrick themes. So, i wanna know, which’s and/or where’s the php file determining the matter and how to that the themes is i’m currently using doesn’t change to the default/kubrick themes in situation like that.