The default color scheme of the editor of GNU Octave's GUI, for some reason, depends on whether the system is using light or dark mode.
If the system is using light mode, then the default color scheme will be correct: either white background with dark text, or dark background with light text.
If, however, the system is using dark mode, then some elements of the default color scheme will be switched, but not others: that means you'll get a white background with light text, or a dark background with dark text.

Steps To Reproduce

Steps to reproduce the behavior:

• Put your desktop environment in dark mode (I used KDE Plasma (both X11 and Wayland) with Breeze Dark)
• Open Octave's GUI
• If it's not the first time opening Octave, reload the default style:
  • In the top bar, Edit > Preferences
  • Click on Editor, scroll down to the almost bottom, and click "Reload default styles"
  • Apply
• In the main window, go to the Editor
• When writing text, you'll see that it's dark with a dark background.
  • If you checked "Second color mode (light/dark)" (and "Reload default styles") in the settings, then it's light with a white background.

Expected behavior

The system theme should either change the color style completely or not at all, instead of changing it partially and creating a poor contrast.

Additional context

This only affects loading the default color scheme: when already applied, changing the system theme changes nothing in Octave.
A workaround to this issue is thus setting the system theme to light, applying the desired color scheme in Octave, and then switching back to the system dark theme.