MacOS Mojave, released in September of last year, introduced a user-toggleable "Dark Mode". This enables a system-wide dark colour scheme, intended to make things easier on your eyes at night, or less distracting when working with content. The mode was supported by all built-in Mac apps on release, but 3rd party applications (including those developed with PyQt) were left looking a bit out of place.
Early implementations had a few issues (see below) but as of PyQt 5.12.2 it's looking great. The pictures below show the same example app (just a random assortment of widgets) under Dark Mode and (default) Light Mode on MacOS Mojave.
pip3 install pyqt5==5.12.2
If you're on PyQt 5.12.2 or over Dark Mode will work out of the box.
Issues on earlier versions
While PyQt 5.2.12 applications look great in Dark Mode, earlier versions have had a few issues. Below are a few screenshots of the same example app taken across earlier releases. If you're releasing your PyQt 5 apps to MacOS you might want to take a look at how it handles.
Downgrading to PyQt 5.12 produces the UI below — more or less fine, but with a missing colour on the spin-wheel nubbin. It doesn't effect the functioning of the app, just looks a bit less nice.
pip3 install pyqt5==5.12
Downgrading further to PyQt 5.10 produces this disaster. Strangely the wheel is now shaded properly, but the text is white-on-white.
pip3 install pyqt5==5.10
Weirdly, although PyQt 5.11 looks just as bad, 5.9 looks slightly better (as in potentially usable).
pip3 install pyqt5==5.9
The bottom line is: if you're targeting MacOS with your applications and using < PyQt5.12.2 then now would be a very good time to upgrade and ensure your app looks as great as it can under Mojave Dark Mode.