Fix to source code

Summary:
A code review shows some minor interesting fixes to do .

• use "" for charset in SCSS *
• optimize background CSS declaration *
• use {{root}} to mark website root in templates
• add some blank lines

(*) Detected through WebStorm code inspection

Test Plan: Check code correctness in Firefox and Chrome

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D540