Make colorscheme.ts a module, instead of a classic script

[louischan-oursky]

In #2058 (comment) we worked around FOUC by making colorscheme.ts a classic script. But the logic there is now needed to be reused. A classic script cannot import code for reuse. This issue is about

• Reuse the logic in colorscheme.ts, by making it a module.
• Make sure there is no FOUC with this change

[linear]

DEV-1644 Make colorscheme.ts a module, instead of a classic script