Thanks
BR. I'm taking @BeckyEBook's advice. I found a nice dark Sigil theme called
QDarkStyleSheet/qdarkstyle/style.qss on GitHub and I've renamed it to "qt_styles.qss" and have been trying to add it into the main Sigil Preferences folder(not plugin_prefs). I've searched my C: drive for 'preferences' and 'prefs' and can't find the main Sigil Preferences folder anywhere on my Windows PC.
Does anyone know the full path to the Sigil Preferences folder on a Windows 7 PC ?