HighContrast: Fix missing background of flat entries in treeviews

With the HighContrast theme, when renaming a file in caja, the background of the text entry is currently transparent, thus getting a black background, and thus text printed black on black, thus unreadable. This makes it explicitly white to get black on white.

Similarly, the inverse HighContrast gets white on grey, which is not so readable, so better make background explicitly black to get white on black there.