The problem is that W32 only has "tabs above the notebook" style (and maybe
"tabs below the notebook", though i don't remember seeing it in use).
Drawing these W32 theme parts on tabs that are located on the left or on the
right side results in obvious, jarring ugliness.

Notebook is one of the worst widgets as far as W32-lookalike styling goes,
as GTK implementation mismatches W32 implementation in a number of ways
(which is why i gave up on making a classic theme for the notebook widget).
In the end i just looked at the way W32 styles notebook tabs and re-created
this with gradients and hardcoded colors. This looks sufficiently OK.

I left the previous (broken) style in XP-specific CSS file, since on XP
notebook tabs must look different, and i'm not about to make another such
style specifically for XP. So on XP tabs-on-top notebooks look OK and other
orientations are still broken.
If needed, MSW7 notebook style can be moved into gtk-win32-base.css after all,
which will apply it to XP too, trading off XP theme conformance for
non-brokenness.