I want to change the default colour for kolibri. What are the procedures please?
As I mentioned in the other thread, doing a search in the codebase for
theme should give you some pointers of where the theme gets initialized: https://github.com/learningequality/kolibri/search?q=theme
Should I work on kolibri design system to modify themes?
No, Kolibri Design System contains a default version of the theme, but it can be dynamically set, as you can see from the Kolibri code paths I linked above.