Polo Beta 10 has an option to select the GTK theme for the UI. Any theme installed on the system can be selected, and the theme is applied immediately (no need to restart the app).
A copy of Arc-Darker theme will be included with Polo, and used as the default theme. It adds only 500 kB to the package size, and ensures that the UI looks consistent on different systems. You can change this from the Settings window if you prefer another theme.