Hi avelino,
This is an issue that I also encountered when working on my dark theme Predawn. I managed to largely solve it, through a lot of hacks. You could try to install Predawn dark theme. If this theme does work as you want for your test case, you could have a look at these two topics, where I tried to explain what I did (+ some valuable comments of bengotow):
Bug: Dark text using dark themes · Issue #1928 · Foundry376/Mailspring · GitHub
Dark theme vs. Theme Starter version · Issue #5 · Foundry376/Mailspring-Theme-Starter · GitHub
Hope that helps!