Plz remember that Apple tried the "tabs within titlebar" thingy for some time and they got a bunch of bad criticisms for it. People mainly had problems hitting the titlebar correctly for dragging the windows. We should not try to get into that area - just like the border of the plate belongs to the customer (and should not get messed up with food).
Respective to the dark/light question, I find myself switching between slate and my modified tango color schemes according to the environmental light level (when the sun shines it's easier to read black on white and vice versa). So I would want both variants
In addition to that I prefer to have the sidebar on the left so that the text area is in the middle of the screen when in fullscreen mode.
One question I miss in every discussion is: How should it look like on other systems than windows? For example the dark sidebar above looks quite awful within OSX using a light color scheme.
I really hope that Jon improves and documents the theme area properly so that we can have proper native (including native scrollbars or look-alike) and negative themes (with cool custom gfx).