Back in Dec 2017 we considered raising the min required version of gtk to 3.20, but ultimately I raised to just 3.18. I'd like to now raise that to 3.20 for LibreOffice 7.1 https://gerrit.libreoffice.org/c/core/+/103901 mostly so we can remove a much unloved complicated hack to support two variants of themeing. Any concerns, let me know.