Two changes would make this app feel more like it was designed for macOS Tahoe.

The first change is updating the corners of the app to match the new more-rounded corners of other apps on macOS Tahoe. To see what I mean, compare the rounded corners of a native app like Safari to the rounded corners of Soulver.

The second change is supporting a dark mode icon. Right now, Soulver stands out on my dock as being one of two icons that don't support dark mode.