Switch to REM, add css var for font-size & colors

[pierregradelet]

Less changes

• Rename .R to .propertyViewport
• Remove less vars for background & colors and set it to css var
• Add .propertyRem mixin to use rem & include media query in one go
• Extract args in .propertyViewport to allow use of args like 10 10 10 or 10 auto 20 in margin / padding properties

[willybrauner]

@pierregradelet I have updated variables folder and files struct

src/atoms
- src/atoms/variables-css.less
- src/atoms/variables-less.less