Refactor SCSS for GB

[strogonoff]

Alongside SCSS refactor, this introduces a few appearance/behavior changes in order to fix the pre-existing issues:

• Styling of the “to top” button
• Margins around cover page
• Nav sidebar & its toggle appearance & behavior consistency with other flavors

Positioning of elements within documnet cover/title page is not changed.

In addition to SCSS, JS controlling sidebar toggle behavior had to be altered, bringing it in line with other flavors (except some measurements, sidebar is narrower in GB).

Screenshots

Wide viewport

Before

After

Narrow viewport

Before

After

[ronaldtse]

Great work @strogonoff , thank you!