Copy the folder breeze-gruvbox-dark to the ${XDG_DATA_HOME}/yakuake/kns_skins (usually ~/.local/share/yakuake/kns_skins)
Copy the folder breeze-gruvbox-dark to ${KDEHOME}/share/apps/yakuake/kns_skins (usually ~/.kde4/share/apps/yakuake/kns_skins or ~/.kde/share/apps/yakuake/kns_skins)
Configure Yakuake -> Appearance -> Get New Skins and search for "Breeze Gruvbox Dark"