When you activate, say, a checkbutton, it briefly becomes "selected", and gets
the appropriate "selected" background color (on W32 that's a shade of blue).
This is abnormal.

The way you activate does influence the display. If you hover your mouse over it
and activate via changing the keyboard focus and pressing <space>, the whole checkbutton
gets selected (which is also abnormal, but at least it doesn't look ugly).
If you do the same, but leave the mouse cursor outside the checkbutton, you get
selection applied only to the checkmark (which is abnormal AND ugly).

The reason, i believe, why this is happening is this style in gtk-win32-base.css:

> *:selected,
> *:selected:focus {
>     background-color: @selected_bg_color;
>     color: @selected_fg_color;
> }

Now, there may be a good case to be made for not using such blanket styles
(as someone on StackOverflow pointed out, they aren't called "Cascading"
Style Sheets for nothing: style inheritance is a feature, and should be used
consciously and not fought against), but unless someone volunteers for the job,
i'd rather just make a quick fix for the issue at hand by overriding the
selected style for check/radio marks.