PixInsight does not support dark mode on Mojave. Sorry, I forgot to mention this in the announcement post.
I'll release a new version that opts out the dark mode tomorrow. It's
a simple keyword in the info.plist that I also forgot to include in the 1.8.6 release. Sorry for this.
To support the dark mode, I have to redesign the application's style sheet from scratch. It is a complete rewrite that may take a very long time. Frankly, I have many priorities before doing this, so don't expect dark mode support in PixInsight in the short-medium term.