Apply the check/radio-button style to GtkTreeView, so that
GtkCellRendererToggle picks the right check/radio-button graphic.
Otherwise, it falls back to a graphic that depends on the system
theme, and which may not play well with the current theme colors.
Note that this only affects the Dark theme; in the Light and Gray
themes, one of the KDE-hack styles "takes care" of this, but this
commit applies the same change to these themes anyway.