Well, no one said anything, but I'll add more information
This is what happens to form fields, even if you apply other colors though usercontent.css.
As you can see (enlarge if needed), some things happen:
1.- the buttons are taking the text color definition (set on usercontent.css to black (#000000)) but the buttons are taking the 3D elements color defined on Windows Scheme.
2.- the textarea elements take the color text (even not shown here) to black, but the area color is taken from windows color scheme (usercontent.css set to #8F9DBF).
3.- the options drop down menu items (on the right) have the same problem with the first option, the selected one by default, but as you note, the elements/options inside the drop down menu are, indeed, taking the values from usercontent.css, that is, black text and #8F9DBF background color.
Is this a GRE issue? Is this a K-meleon 1.6b2 issue?
In a few days, if no one confirms anything, I'll post as a bug.