I updated to macOS v11.6 (Big Sur) and Safari 15.0 yesterday, and now Stack Exchange websites are defaulting to a "dark" toolbar on my machine. "Light" mode is set the system default under System Preferences, so this is strange.
This behavior only seems to occur on Stack Exchange sites; other websites are unaffected. All Stack Exchange sites that I have looked at are affected except for Stack Overflow, which remains in "light" mode.
Is this a problem on my machine, a bug in the Stack Exchange website code, a bug in Safari 15.0, or something else?