Origin provide 6 build-in dark themes: Black, Blue, Coffee, Grey, Pumpkin, Purple from Preferences: Dark Theme menu.
To create user-defined dark theme:
Choose Help: Open Folder: Program Folder.
Go to \Themes\DarkMode\ subfolder and copy any ini file there.
Choose Help: Open Folder: User Files Folder.
Go to \Themes\DarkMode subfolder. If no DarkMode folder there, create it.
Paste the ini there.
Rename it e.g. MyGray.ini.
Open it in Notepad to edit the colors. Most entries are self-explanatory. E.g. modify the workspace background color to light gray, and show red as active widow indicator.
WorkspaceBackground=#999999
ActiveWindowIndicator=#FF0000
Select Preferences: Dark Theme: MyGray (user) to see effect.
Every time making some change in ini file, save the file and click the toggle GUI button on Dark Mode toolbar twice to see updated effect.