CSS of 'Menu' block doesnt get inherited

Heya,

When I place a customized Menu Block in the Menu region (horizontal navigation), the customized Menu Block appears as a plain horizontal menu without the expected markup. Strangly enough, the CSS of the collapsing submenu is kept intact. When I move standard Drupal blocks into the menu region such as Secondary Links, everything is okay. I really think its Menu Block that's not correctly integrating in the theme.

Get what I mean?
Thanks