Currently you have to manually switch between Light theme and Dark theme.
Changing the theme automatically can be done in many different ways. I think the best approach will be the user choosing a time period for when a particular theme to be chosen. For example, from 7 AM to 7 PM, Mailspring will be using a Light theme, and from 7 PM to 7 AM, Mailspring will be using a Dark theme.
Another way will be to look at the system theme and deciding based on that. For example, on Windows, Mailspring can check a Registry value called “AppsUseLightTheme” located at “HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize” And if it’s set on 0, that means Windows is using Dark Mode, and if it’s set to 1, that means Windows is using Light mode.