Remove rems unit from v5

[Amantini1997]

Description

Everywhere across the app we use px as unit to define components outlook.
However, in theme, we use rems for font-size and line-height.
It would be nice to use the same unit everywhere

Suggested solution

Refactor theme and convert rems to pxs values

Goal

• Use a consistent approach for units
• Keep the more intuitive unit only to make it simpler to work with

Tasks

• Refactor the theme file (and any other v5 files if any) to only use px