For anyone who was using K-9 wondering why Thunderbird looks no different, it's because they aren't different.
They build both apps from the exact same codebase. Only difference between the two are the default color scheme, the branding icons, and the text strings of the application's name. It's literally just a choice of which brand skin you prefer.
Which, honestly, kinda cool. A virtually zero-cost way to keep a few K-9 stans happy.