Sadi estava certo.
Então, em metacity-theme-1.xml , nos dois blocos abaixo, adicionei has_title="false"
e alterei alguns valores. E voilà.
<frame_geometry name="frame_geometry_normal" has_title="false" title_scale="medium" rounded_top_left="true" rounded_top_right="true" rounded_bottom_left="false" rounded_bottom_right="false">
<distance name="left_width" value="1"/>
<distance name="right_width" value="1"/>
<distance name="bottom_height" value="1"/>
<distance name="left_titlebar_edge" value="1"/>
<distance name="right_titlebar_edge" value="1"/>
<distance name="button_width" value="0"/>
<distance name="button_height" value="0"/>
<distance name="title_vertical_pad" value="1"/>
<border name="title_border" left="0" right="0" top="0" bottom="0"/>
<border name="button_border" left="0" right="0" top="0" bottom="0"/>
</frame_geometry>
<frame_geometry name="geometry_maximized" has_title="false" rounded_top_left="false" rounded_top_right="false" rounded_bottom_left="false" rounded_bottom_right="false">
<distance name="left_width" value="0"/>
<distance name="right_width" value="0"/>
<distance name="bottom_height" value="0"/>
<distance name="left_titlebar_edge" value="0"/>
<distance name="right_titlebar_edge" value="0"/>
<distance name="button_width" value="0"/>
<distance name="button_height" value="0"/>
<distance name="title_vertical_pad" value="0"/>
<border name="title_border" left="0" right="0" top="0" bottom="0"/>
<border name="button_border" left="0" right="0" top="0" bottom="0"/>
</frame_geometry>