@snow On mastodon.social, Mastodon's default theme applies, which is the dark theme. You can change your theme on both the sites to whichever you prefer.

Mastodon.social fetches information of your profile from qoto.org, but everything on QOTO – like your profile's metadata – is not of the same format as in mastodon.social; QOTO has been modified to some extent. This creates some compatibility issues in federation. And then, it takes some time for one instance to fetch all the data from another instance.