If a persona is chosen for the panel area, the icons are badly visible, because the persona is fully visible under the buttons in the top panel.
The icon buttons should not be 100% transparent, 20% to 50% should be ok.
I guess it depends on which theme you are using.
I have the same problem under Ubuntu 15.04 with Ambiance theme and LO 5.0.x when GTK3 vcl plugin is used.
Best regards. JBF
*** This bug has been marked as a duplicate of bug 89760 ***
@Xisco: This isnt a duplicate.
I don't think that icon themes can be expected to adjust according a proprietary UI solution. It's up to the icon designers to define a proper theme - and to the user to chose something that fits into his or her looks and feel. If we go with less transparency we spoil the background colored by the system theme.