From the site: "Registry Finder automatically uses dark theme when you turn on dark mode in Windows. (Head to Settings > Personalization > Colors. Select the "Dark" option under the "Choose your app mode" section.) Dark mode is available only in Windows 10 so, in turn, the dark theme in Registry Finder is available only in Windows 10."
Hmm... Perhaps, this requires particular update of Windows. What version of Windows do you have? (Please start command prompt, type "ver" and copy&paste the output.).