Whether it's a custom  master page  or not. If you edit one of the styles
stored in core.css using SPD it will save a modified copy  of core.css to
the _styles directory of the site. If you look at the master  page you
will see in the html header that the core.css style  sheet is actually
loaded using a custom server control called CSSlink. This control will
load the core.css from the 12 hive if it doesn't find a core.css in the
local _styles directory, otherwise it loads the local  one.