OK, I guess nobody knows the answer. Still frustrated, I dug a little deeper. It turns out the tabs along the top of the window and other UI elements are controlled by PNG files here, at least on Windows 7:
C:\Users\YOURNAME\AppData\Roaming\Sublime Text 2\Packages\Theme - Default
I edited tab_selected.png to make it a sort of grayish green. Now it contrasts well with the other tabs.
Other elements of the UI colors, etc, seem to be here in "tmTheme" files:
C:\Users\YOURNAME\AppData\Roaming\Sublime Text 2\Packages\Color Scheme - Default
which are plain text files and can be edited...