SimpleMind can show mind maps in Dark Mode. The built-in style sheets support dark mode, meaning if you switch to dark mode the style sheet will as well.
For example, this is a mind map in Light Mode:
Dark mode - Mind Map in Light Mode
And this is the same mind map in Dark Mode:
Dark mode - Mind Map in Dark Mode

Mac: By default when Mac is in Dark Mode, SimpleMind will be as well. You can change this via the main menu "SimpleMind"-"Preferences" or turn this off/on for stylesheets only by clicking the options wheel in Stylesheets.

Windows: Change to dark mode via the main menu "SimpleMind"-"Preferences". Or change the stylesheets only in Stylesheets, next click the options wheel and select Dark Mode.

iPad&iPhone and Android: When your device is in Dark Mode, SimpleMind will be as well. You can turn this off/on for stylesheets via the options wheel in Stylesheets.