Step 1: Find your ~/.idlerc folder
Based on what I found on StackOverflow, it's pretty easy to find it on your home directory. Finding it on Mac is a bit different(That's what I will talk about next).
Open the terminal in your root directory then type-
$ cd ~/.idlerc $ touch config-highlight.cfg $ open .
Step 2: Edit the config-highlight.cfg file
This will open the ~/.idlerc folder with the custom theme config file. Now open this file from finder or use
$ open -a TextEdit config-highlight.cfg
Step 3: Update the theme on IDLE
Now paste the theme code snippet into this file and save it. It's simpler from here on. Open IDLE Preferences, go to highlights, and choose the custom theme.