Update color and background-color utilities

- Adds new functions to generate additional Sass maps
- Adds new root variables for rgb() versions of our theme colors, plus a few extras
- Adds ability to change the alpha transparency of text color and background color utilities with new utilities, inline styles, or local CSS var
- Updates documentation for color and background-color utilities pages
- Deprecates .text-black-50 and .text-white-50 since those (and additional transparency levels) can be generated on the fly

Change approach slightly to prevent cascade

2 files changed, 95 insertions, 0 deletions

diff --git a/site/content/docs/5.0/utilities/background.md b/site/content/docs/5.0/utilities/background.md
index 7b1b481401..d5edbdf3b3 100644
--- a/site/content/docs/5.0/utilities/background.md
+++ b/site/content/docs/5.0/utilities/background.md
@@ -35,6 +35,44 @@ Do you need a gradient in your custom CSS? Just add `background-image: var(--bs-
 {{< /colors.inline >}}
 {{< /markdown >}}
 
+## Opacity
+
+<small class="d-inline-flex px-2 py-1 font-monospace text-muted border rounded-3">Added in v5.1.0</small>
+
+As of v5.1.0, `background-color` utilities are generated with Sass using CSS variables. This allows for real-time color changes without compilation and dynamic alpha transparency changes.
+
+### How it works
+
+Consider our default `.bg-success` utility.
+
+```css
+.bg-success {
+  --bs-bg-opacity: 1;
+  background-color: rgba(var(--bs-success-rgb), var(--bs-bg-opacity)) !important;
+}
+```
+
+We use an RGB version of our `--bs-succes` (with the value of `25, 135, 84`) CSS variable and attached a second CSS variable, `--bs-bg-opacity`, for the alpha transparency (with no default value, but a fallback of `1`). That means anytime you use `.bg-success` now, your computed `color` value is `rgba(25, 135, 84, 1)`.
+
+### Example
+
+To change that opacity, override `--bs-bg-opacity` via custom styles or inline styles.
+
+{{< example >}}
+<div class="bg-success p-2 text-white">This is default success background</div>
+<div class="bg-success p-2" style="--bs-bg-opacity: .5;">This is 50% opacity success background</div>
+{{< /example >}}
+
+Or, choose from any of the `.bg-opacity` utilities:
+
+{{< example >}}
+<div class="bg-success p-2 text-white">This is default success background</div>
+<div class="bg-success p-2 text-white bg-opacity-75">This is 75% opacity success background</div>
+<div class="bg-success p-2 text-dark bg-opacity-50">This is 50% opacity success background</div>
+<div class="bg-success p-2 text-dark bg-opacity-25">This is 25% opacity success background</div>
+<div class="bg-success p-2 text-dark bg-opacity-10">This is 10% opacity success background</div>
+{{< /example >}}
+
 ## Sass
 
 In addition to the following Sass functionality, consider reading about our included [CSS custom properties]({{< docsref "/customize/css-variables" >}}) (aka CSS variables) for colors and more.
@@ -63,6 +101,14 @@ Grayscale colors are also available as a Sass map. **This map is not used to gen
 
 {{< scss-docs name="gray-colors-map" file="scss/_variables.scss" >}}
 
+RGB colors are generated from a separate Sass map:
+
+{{< scss-docs name="theme-colors-rgb" file="scss/_variables.scss" >}}
+
+And background color opacities build on that with their own map that's consumed by the utilities API:
+
+{{< scss-docs name="utilities-bg-colors" file="scss/_variables.scss" >}}
+
 ### Mixins
 
 **No mixins are used to generate our background utilities**, but we do have some additional mixins for other situations where you'd like to create your own gradients.
diff --git a/site/content/docs/5.0/utilities/colors.md b/site/content/docs/5.0/utilities/colors.md
index 266f671f6d..9502c48777 100644
--- a/site/content/docs/5.0/utilities/colors.md
+++ b/site/content/docs/5.0/utilities/colors.md
@@ -23,10 +23,51 @@ Colorize text with color utilities. If you want to colorize links, you can use t
 <p class="text-white-50 bg-dark">.text-white-50</p>
 {{< /example >}}
 
+{{< callout warning >}}
+**Deprecation:** With the addition of `.text-opacity-*` utilities and CSS variables for text utilities, `.text-black-50` and `.text-white-50` are deprecated as of v5.1.0. They'll be removed in v6.0.0.
+{{< /callout >}}
+
 {{< callout info >}}
 {{< partial "callout-warning-color-assistive-technologies.md" >}}
 {{< /callout >}}
 
+## Opacity
+
+<small class="d-inline-flex px-2 py-1 font-monospace text-muted border rounded-3">Added in v5.1.0</small>
+
+As of v5.1.0, text color utilities are generated with Sass using CSS variables. This allows for real-time color changes without compilation and dynamic alpha transparency changes.
+
+### How it works
+
+Consider our default `.text-primary` utility.
+
+```css
+.text-primary {
+  --bs-text-opacity: 1;
+  color: rgba(var(--bs-primary-rgb), var(--bs-text-opacity)) !important;
+}
+```
+
+We use an RGB version of our `--bs-primary` (with the value of `13, 110, 253`) CSS variable and attached a second CSS variable, `--bs-text-opacity`, for the alpha transparency (with no default value, but a fallback of `1`). That means anytime you use `.text-primary` now, your computed `color` value is `rgba(13, 110, 253, 1)`.
+
+### Example
+
+To change that opacity, override `--bs-text-opacity` via custom styles or inline styles.
+
+{{< example >}}
+<div class="text-primary">This is default primary text</div>
+<div class="text-primary" style="--bs-text-opacity: .5;">This is 50% opacity primary text</div>
+{{< /example >}}
+
+Or, choose from any of the `.text-opacity` utilities:
+
+{{< example >}}
+<div class="text-primary">This is default primary text</div>
+<div class="text-primary text-opacity-75">This is 75% opacity primary text</div>
+<div class="text-primary text-opacity-50">This is 50% opacity primary text</div>
+<div class="text-primary text-opacity-25">This is 25% opacity primary text</div>
+{{< /example >}}
+
 ## Specificity
 
 Sometimes contextual classes cannot be applied due to the specificity of another selector. In some cases, a sufficient workaround is to wrap your element's content in a `<div>` or more semantic element with the desired class.
@@ -57,6 +98,14 @@ Grayscale colors are also available as a Sass map. **This map is not used to gen
 
 {{< scss-docs name="gray-colors-map" file="scss/_variables.scss" >}}
 
+RGB colors are generated from a separate Sass map:
+
+{{< scss-docs name="theme-colors-rgb" file="scss/_variables.scss" >}}
+
+And color opacities build on that with their own map that's consumed by the utilities API:
+
+{{< scss-docs name="utilities-text-colors" file="scss/_variables.scss" >}}
+
 ### Utilities API
 
 Color utilities are declared in our utilities API in `scss/_utilities.scss`. [Learn how to use the utilities API.]({{< docsref "/utilities/api#using-the-api" >}})