Compiling Libreoffice <5.3 with USE=gtk3 causes some GTK3 themes to not display properly, causing white borders between the rows in Libreoffice's toolbar. See: https://github.com/horst3180/arc-theme/issues/659 Libreoffice's team released a patch for it here: https://cgit.freedesktop.org/libreoffice/core/commit/?h=libreoffice-5-3&id=d2514abf43f85aae48d2d26da7a1871383841cca I tested it with =app-office/libreoffice-5.2.3.3-r1 and it fixed the problem for me (tested with both arc and numix)
It's fixed in 5.2.6.2, so I'm going to close this bug