@mjktfw, Sorry for the late response. We are going to work on an option that would allow you to apply the editor background color from a Color Theme.
Regarding the CSS Customization you should be able to contribute CSS Styling directly to our darkest dark theme by using the org.eclipse.e4.ui.css.swt.theme extension poit. The Darkest Dark theme ID is com.genuitec.eclipse.themes.dark.
example:
<extension
point="org.eclipse.e4.ui.css.swt.theme">
<stylesheet
uri="css/your-css-file.css">
<themeid
refid="com.genuitec.eclipse.themes.dark"></themeid>
</stylesheet>
</extension>
Please let me know if you have any concerns,
Cheers!
Aaron Lara