Open the Preference Menu, by clicking "Edit->Preferences.." as illustrated above.
In the Preference Menu; the color options are available in the first menu.
Use the drop-down menu from "GUI Theme" and select "Dark Theme".
Confirm your choice by clicking "OK"
The GUI should now look similar to this
For additional information, visit the release notes for 2020.10 at: https://resinsight.org/getting-started/whats-new/releasenotes_2020_10/