When started in dark mode, Polyphone for macOS does not properly support dark mode. This makes the UI hard to read. It does seem to use the dark mode background color but the foreground color of UI elements and the menu is always black. When macOS is in light mode, Polyphone uses the light background color.
In the best case the design settings should support detecting changes in the system mode and adopt mode colours accordingly.
There are themed templates in settings but the default setting depends on the system dark/light mode. In dark mode the default setting has grey background colors and a black button color.
With this project being developed on a voluntary basis it is acceptable to just fix the default setting to ignore dark/light mode and use fixed colors. Other color themes already have fixed colors.