Thanks for your interest in Kolibri! Searching the codebase for theme should give you some pointers of where the theme gets initialized: Search · theme · GitHub
One thing to be wary of, is that we have carefully selected the colours used in Kolibri in order to support best practices in accessibility. Our accessibility statement is available to read, and I would suggest paying particular attention to the WCAG guidelines for making text and other elements distinguishable, paying particular attention to use of colour, and minimum contrast standards.
Kind Regards,
Richard