Feat/convert mdbook/154

[mehtaishita]

Problem

There are multiple mdbook themes available, but we want to simplify it and only have two (light and dark) that match our current spec site.

154

======== We took current colors of the spec and changed the values in variables.css

Change summary:

• [x] Changes to variables.css
• [x] Keeping two theme options in theme.hbs

Steps to Verify:

1. Build spec
2. Compare with current spec site

Screenshots (optional):

[mehtaishita]

The random switching of themes when navigating to "DSNP Implementations spec" etc happens when I build the mdbook locally, but not when I serve. I believe that issue is not related to the change of variables - but how @media (prefers-color-scheme: dark) { .light.no-js { ... } in variables.css values are honored in the browser.

I'll open a bug to handle those changes.

[saraswatpuneet]

@mehtaishita based on your comment is it when you serve the book this issue does not appear about random dark theme?

[saraswatpuneet]

Merging on behalf of @mehtaishita