From the Linux Mint blog (https://blog.linuxmint.com/?p=4675 https://blog.linuxmint.com/?p=4675), it seems this is the crucial issue. "They will integrate well

From the Linux Mint blog (https://blog.linuxmint.com/?p=4675 https://blog.linuxmint.com/?p=4675), it seems this is the crucial issue. "They will integrate well. If we let you choose a desktop theme, ALL installed applications will support it." So it seems like these Xapps will be built for Mint's desktop themes instead of the Gnome design language. Hopefully the work will be worth it for the Mint project.