To apply a dark theme to the Node-Red nodes workspace, simply do the following.
1) Download the library to your computer.
“npm install @node-red-contrib-themes/midnight-red”
2) Add it to the editorTheme section of your Settings.js located in the Node-Red main folder and then restart Node-RED.
The interface seen when we restart Node-Red
You can visit this page to use many other themes. https://www.npmjs.com/~mbonani