Ensure to pass 4.5:1 contrast ratio everywhere

[ffoodd]

Sounds like dependabot one, but isn't :)

Paving the way in #30468 — and to keep going with #29315 : my goal is to make this happen after tackling every obstacles.

To solve before the bump

Those should probably adressed separately, consider this as a roadmap to enlightment :D

• Ensure to increase contrasts on buttons & colored links hover/active states #30989 → .btn-success:hover case mentionned in #30468 (comment);
  • Update green to pass 4.5:1 #30972 is maybe the wrong direction too—there mght be no need to change colors at all.
  • v5: Tweak some colors #31276 did it 🎉
• Colors rewrite #30622 → color-level(), lighten() and darken() seem misused: either on :hover for buttons (always darken) or to define color and background-color with the same level, no matter what the base color is (eg. .alert-light seems wrong) — there's probably room to improvement by using tint() and shade() here;
  • Relates to Review *-bg-level and *-border-level variables #30487
• Bump $dark to $gray-900 #30550 → .text-info, .text-black-50 and .text-white-50 can't make it to 4.5:1 — however, using $gray-900 instead of $gray-800 for $dark would solve this (and several other dark-ish things with insufficient contrasts);
• A few components use light grays as background (mostly in cards and navbars) without accounting for it in their color usage, resulting in insufficient contrasts when used with .text-muted or .disabled things;
• Some .btn-outline-* and .link-* examples should be shown with dark background:
  1. Contrast colors added to theme-colors #30044 will help to associate a contrast color for the docs part, through the data layer
  2. Outline buttons & colored links: set bg-dark where appropriate #30551 was a wrong direction but helped to define a visual consideration for examples with multiple backgrounds
  3. TODO will need another PR after Contrast colors added to theme-colors #30044 to tackle this, I guess.

Merging this would be a kind of consecration 🎉

Fixes #25126

[ffoodd]

This PR itself can now tackle insufficient contrasts through our codebase and docs, now that #31276 bumped $min-contrast-ratio to 4.5 🎉

[XhmikosR]

@ffoodd is this still valid or can we close it?

[ffoodd]

We can close it. If we're still missing some contrast out there, we'll tackle it later.