I would really like a sepia type theme like Solarized or Gruvbox light. I tried to make my own but then broke mailspring so badly i had to uninstall it and then also delete all the registry settings. 
As far as I know, Mailspring does not offer a built-in sepia or yellow theme, but you can try to create your own custom theme using the Mailspring theme editor.
To use the theme editor:
- Open Mailspring and go to the “Appearance” tab in the Preferences window.
- Click on the “Edit Theme” button.
- Use the various options in the theme editor to customize the colors and styles of the Mailspring interface. You can choose from a variety of predefined colors, or enter your own custom colors using hex codes.
- When you’re happy with your custom theme, click the “Save” button to apply the changes.