C# css & master pages

I have a C# 2010 web forms application that uses master pages, themes, and cascading styles sheets on the same web pages. Since I am new to master pages, can you tell me how I would know what part of the web pages uses css and what part uses master pages, and the base master page, and what part uses themes? If there some kind of a rule on what takes precedence if the same part of the master page is access? Does css, theme,and or master page precendence. If there is no precendence, can you tell me how I would knowwhat part uses css, what part uses themes, and what part uses themes?