Issue Summary

The Dark Theme was launched starting with the Jira Software Data Center 10.3 release;

• JRASERVER-63150 - A dark theme for Jira Server - dark background, contrasting text, etc

However, the customization of the color scheme is still not available for Dark Theme. 

Steps to Reproduce

1. Enable Dark theme in Jira > Profile > Dark
   
2. Go to System > User Interface > Look and feel
   You will see that customizing the color scheme is not yet supported;
   

Expectation

The color scheme configuration should also be applicable to Dark Theme.