Hey, Mailspring / other mail client team, I found out your little secret, thanks for spending my time solving mystery on why the heck my dark email becomes white and shiny?!

Solution

In your body tag or in your .css add:

  body {
    filter: none !important;
  }
  img {
    filter: none !important;
  }

Next time your email client will do this shit, please take a look into the source code (Ctrl + Shift + I) and search for filter or something like that, usually inline styles will overlap any other styles and thing will work

ENJOY!