Is your feature request related to a problem? Please describe.

No. It is a feature request.

We can define a bunch of themes globally from which the user can select any theme of choice and add it in the config.yml file.

The globally define theme can include all the design elements: header, footer, buttons...

Describe the solution you'd like

What cannot be done?

This is my opinion 👇🏻👇🏻👇🏻

We cannot allow users to define their own customized theme where users can change the color of the text or button.

Additional context

I have also discussed about this in the NetlifyCMS community talk on 29-08-2018.