Version: 15.12.3 (from http://archive.ubuntu.com/ubuntu/pool/universe/g/gwenview/gwenview_15.12.3-0ubuntu1_amd64.deb). In fullscreen mode the button "Add Filter" and other buttons has light text and background colors, so text invisible (white on white). Reproducible: Always
Also almost white text and background colors used in rename dialog (key F2), so user can't see typed text.
This is probably an issue with your color theme. With Breeze, it looks perfect for me: white text on a black background.