From the Help online scratchpad: https://docs.google.com/document/d/1ZnSQ4B216CNROHvzXgRwe2SuoJH9MikcxByXYVQm1z8/edit?ts=59cbd4e8# Tab colors - Currently to difficult to see as it's so light, both background and foreground, so simply have the hover-over color as the default and maybe put a darker 1px border color around them File to touch defaults.css
Ilmari Lauhakangas committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/help/commit/?id=50b850013374e0f711b5cded92739934d758bf1f tdf#114709, tdf#112758, tdf#112757