So I accidentally deleted the Elysium theme with the minus button in blender preferences. Is there any way to recover it?
As @MrZak suggested, you can get the files from online resources.
Copy the contents of the XML file for the theme you want
Go to the folder in which blender is installed(usually located in program files)
Navigate to "2.79\scripts\presets\interface_theme"
Create a new XML file and paste the previously copied item in it