Dark mode has been introduced at WWDC 2018. And still there are some applications not supporting it. Amongst them the further excellent Carbon Copy Cloner. Not a cheap app by far. And not the most complex UI either.
It’s no big thing as I hardly open up the app itself, but still a nuisance. And hard to understand why the developer didn’t implement it yet.
Another one that comes to mind is ABBYY FineReader. Arguably the best OCR solution on the Mac. But hardly supported.
Things which look relatively simple aren’t always. Especially if the app has existed for a long time and maybe doesn’t use the latest features made available by apple to developers.
If you’re a user, it’s worth you reaching out to the developer to ask them if they’d consider adding Dark Mode to their app.
Indeed. It’s a great OCR engine, but it’s a not great app, by any stretch of the imagination.
Some apps also don’t support automatic dark mode, which really sucks.
Nobody said it was easy 
The developer of CCC has been telling it’s on their list since, well, since the introduction of dark mode in 2018. That’s over 2 years now, which makes it hard to believe it’s ever gonna happen.
In case of CCC functionality (i.e. a reliable backup) is more important than looks. So that’s why the topic title calls it a nuisance. Still, this is not a 2 dollar app. And I wouldn’t mind paying for an upgrade that brings the whole application up to date with the latest OS on all levels, like e.g. Hazel did (which moved from a pref pane to an application and a background process). So far, the developer of CCC has been addressing all kinds of issues with Apple changing things in their filesystem. For free! And I appreciate that a lot. But the few times a year I open the applications, it would be nice to see it in full dark mode glory.
In case of ABBYY FineReader it’s a bit of a different story. That’s a 1200 people company selling a $120 dollar application. One could expect that to be a bit more up to date. Apart from the great OCR engine (like @tjluoma rightfully says) and Applescript support, there ain’t much to like about this thing. And tbh, if I compare it to Tesseract (backed by Google) it’s hard to justify the expense these days. Even for a small business.