We can’t reproduce this on our systems. You may be surprised to learn that the only thing hat BuddyPress looks for before triggering such messages is the tag ‘buddypress’ at the top of the theme’s stylesheet (style.css).
Other than that it has no way of knowing whether the theme works with BP or not. So in that sense the message means nothing! But as I say the message isn’t appearing for us.
Please check that this tag is intact on Xi and the Dark Child theme.
We do know however that in some cases you can get a similar message from bbPress, but this appears on all themes.
On another note, while investigating this issue we noticed that the Dark child theme didn’t have ‘dark’ styles for buddypress. So there is an update to the theme that tweaks the base theme bp styles to be more inline with the dark theme. But this shouldn’t make any difference to your message.