The fancybox div has a set of styles you can manipulate, look at the .css file included, the classnames are:
div#fancy_overlay
div#fancy_loading
div#fancy_loading div
div#fancy_outer
div#fancy_inner
div#fancy_content
div#fancy_div
img#fancy_img
div#fancy_close
#fancy_frame
#fancy_ajax
a#fancy_left, a#fancy_right
a#fancy_left
a#fancy_right
span.fancy_ico
span#fancy_left_ico
span#fancy_right_ico
a#fancy_left:hover, a#fancy_right:hover
a#fancy_left:hover span
a#fancy_right:hover span
#fancy_bigIframe
div#fancy_bg
div.fancy_bg
div#fancy_bg_n
div#fancy_bg_ne
div#fancy_bg_e
div#fancy_bg_se
div#fancy_bg_s
div#fancy_bg_sw
div#fancy_bg_w
div#fancy_bg_nw
div#fancy_title
div#fancy_title div
div#fancy_title table
div#fancy_title table td
td#fancy_title_left
td#fancy_title_main
td#fancy_title_right
This is as-of the 1.2.6 version.
The big ones you probably want are:
div#fancy_outer
div#fancy_inner
div#fancy_content
div#fancy_div
Does manipulating these styles not accomplish what you're after? If not please clarify a bit more.