Form control color refactoring

[julien-deramond]

Description

While working on #2341, we spotted some inconsistencies and weird values for Sass variables (e.g. https://github.com/Orange-OpenSource/Orange-Boosted-Bootstrap/pull/2341/files).

IMO there's a place for some refactoring to avoid having to do weird calculations for more readability, and maybe fewer bugs in the future.