Unify the styling of components with the same variant

[ffffffelix]

All components (input, dropdown, button) with the same variant (ghost, outlined) must have the same style.
Add scss variable to easily share style cross components.

For example, the Dropdown with the ghost variant should have the same style as the Button with the ghost variant.

Could you also check if we have other incoherences like that?

Idea:

• Create a scss placeholder %ghost that can manage all states (:hover, :active, :focus) that can be used for all ghost components