Not really. Initial Dark mode support was not added until Sigil 1.1.0 or so. There may be a way to use Qt qss files to try to emulate a dark mode in earlier versions but my guess is that would have lots of minor nits and issues.
I do remember someone did a Qt darkmode qss file at some time point and posted a link on MobileRead but that was years and years ago. Perhaps someone else here has a link to it.
Here are some a google search returned:
https://github.com/ColinDuquesnoy/QDarkStyleSheet
and
https://github.com/EClaesson/QTDark
All that said, I truly recommend not using a version of Sigil that is so very old. That version of Webkit it uses is very very insecure and not something I would run in our current networked world. Ransomeware and data lossage is just too costly to not try and protect yourself.