Colors rewrite

[MartijnCuppens]

Purpose of this PR is to use a uniform approach on using colors:

• Use tint-color() to replace lighten()
• Use shade-color() to replace darken()
• Use scale-color() to replace color-level() (since we don't work with levels anymore)
• Get rid of $theme-color-interval, the 8% levels are hard to calculate with.
• Table variants, list item variants & alerts variants used to have different colors:
  • https://twbs-bootstrap.netlify.app/docs/5.0/content/tables/#variants
  • https://twbs-bootstrap.netlify.app/docs/5.0/components/alerts/#examples
  • https://twbs-bootstrap.netlify.app/docs/5.0/components/list-group/#contextual-classes
    vs
  • https://deploy-preview-30622--twbs-bootstrap.netlify.app/docs/5.0/content/tables/#variants
  • https://deploy-preview-30622--twbs-bootstrap.netlify.app/docs/5.0/components/alerts/#examples
  • https://deploy-preview-30622--twbs-bootstrap.netlify.app/docs/5.0/components/list-group/#contextual-classes
    (color contrast on the light variants is still an issue, but can be tackled later on to prevent this PR from being too overwhelming)

[ffoodd]

I really prefer the percentage values you're suggesting to remove $theme-color-interval, it's much clearer!

[mdo]

Is this safe to bring back and set to ready for review for Alpha 3?

[MartijnCuppens]

Yeah, fine by me. Worked a bit on this today, but it seemed to be a bit more complex than I initially thought. I'll keep you posted.

Edit: It's done ;)

[ffoodd]

LGTM, really helpful changes!

[mdo]

Hell yeah, huge refactor and update, @MartijnCuppens!

[stof]

Any reason to implement this change using black and white (essentially working in HWB) rather than using Sass's scale-color($color, $lightness: 12%) ?

[miken32]

A little more guidance in the migration documentation would be helpful when 5.0 is released. Through some experimentation I found that shift-color($color, $level * 8) is similar to color-level($color, $level).