Sigil 0.9.14 is so old there was no support yet for mac Dark and Light modes.
If you feel comfortable in using Terminal.app and changing system defaults on macOS you might try the following command:
defaults write com.sigil-ebook.Sigil.app NSRequiresAquaSystemAppearance -bool yes
I am not sure if that will work or not, but it is worth a try.
Here is a link that may help I found via a google search:
https://www.techjunkie.com/exclude-a...-macos-mojave/