Customize the appearance (theming)

**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.