You can create new system symbols using
env\sys\symbols\template_set, or modify the
existing ones (i.e. wow_set). Open each bitmap 
in a paint program and draw as you like. Remember 
that bottom-left pixel's color will be treated
as transparent color. Each bitmap should not
exceed 20x20 pixel size (better use the
size of each template bitmap).

Symbols meaning : 

wnd_cb		checkbox glyph
wnd_close	window close gadget
wnd_cycle	cycle button glyph
wnd_depth	window depth gadget
wnd_down	arrow down
wnd_left	arrow left
wnd_resize	window resize gadget
wnd_right	arrow right
wnd_up		arrow up

If you create an original style, please send
me a zip archive of the set:it will be included
in the next release of WOW.