I've Started using Dark Modes on lots of applications and websites, when available (such as on Outlook.com and here on azurecurve|Ramblings of a Dynamics GP Consultant. One of the applications I wanted to do this on was Notepad++, but I struggled to find the themes section to make the change.
The problem was that I suffered from a bit of tunnel vision when looking for the option. I was looking for themes, when I should have been looking for styles; specifically the Style Configurator:
At the top of the Style Configurator, there is a Select Theme dropdown list at the top of the window with twenty-something themes available by default:
You can edit the theme if necessary and then click Save & Close. Notepad++ will now display using the selected theme: