This is now a built-in plugin in Porcupine
Plugin to use the Sun Valley ttk theme in Porcupine
Install sv-ttk inside Porcupine's venv
pip install sv-ttk
Copy sun_valley_ttk.py into Porcupine's extra plugins directory.
The ttk_themes plugin might need to be disabled, as sv-ttk can conflict with it.
You can select the theme in the settings dialog. Don't use the UI themes menu for that, as sv-ttk doesn't work that way, and will raise an error.