Create Custom Syntax Colouring in Amazon Cloud9

Amazon AWS Cloud9 IDE by now (2019) is having no feature to create custom syntax colouring, only choose from available ones. But there's a work-around here:

1. Make theme CSS
   Go to some temporary dir:
   git clone https://github.com/ajaxorg/ace
   cd ace/tool
   npm i
   node tmtheme.js THEME-NAME \
   /path/to/some-theme.tmtheme /path/to/output/dir
2. Edit the theme CSS
   Go to /path/to/output/dir
   Open THEME-NAME.css
   Replace the prefix CSS class ".ace-XXX" to ".ace_content"
   Ctrl-A (Select all CSS), Ctrl-C (Copy)
3. Apply the theme CSS
   Open Cloud9 IDE, go to Preferences >> Themes >> Edit your stylesheet
   Ctrl-V (Paste the CSS), Ctrl-S (Save)
4. See it applied!
   Open any file, to enjoy the custome theme.

Reference:
https://community.c9.io/t/how-do-i-upload-a-custom-theme-to-my-workspace/20893/3