Trying to change to another palette. Clicking on palette side button for either font colour or highlight colour to offer alternatives shows the full list including a custom .soc put in ~.config/libreoffice/4/user/config but click to change to another palette no longer has any effect.
Fresh TDF build 5.2.0.4 does not have this bug (nor the rpm flatpak package of the same build) but is currently not as responsive as 5.1.5.2. A partial workaround is to remove the standard.soc from ~/.config/libreoffice/4/user/config of LO 5.1.5.2 and replace with customized standard.soc.
LibreOffice Version: 5.1.5.2,
Build ID: 5.1.5.2-3.fc24,
filetype : .odt,
OS : Fedora 24