Go to the .idlerc folder(on my pc it's location is: 'C:\Users\ MY_PC_NAME \ .idlerc',
and replace 'config-highlight.cfg' file with the file given here, and enjoy new custom themes.
Themes included are:
1.[monokai] -- preferred(gives feel of using sublime Editor)
2.[Obsidian]
3.[desert]
4.[tango]
To apply themes, open 'Idle->Options->Configure IDLE->Highlighting'
Select the 'a Custom Theme' check-Box
Then select the Theme you wish from the second button below it
#Enjoy!!