feat(checkbox): add ionic theme styling (#29335)

Issue number: N/A

---------

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issue. -->

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

Checkbox does not have an ionic theme implementation.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Adds the ionic theme styles and initial features for checkbox. 
- This PR is a combination of all the individual PRs already reviewed.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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  2. Update the BREAKING.md file with the breaking change.
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## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

We should confirm that the visual styles and behavior function as we
expect with all the changes combined in this PR.

---------

Co-authored-by: João Ferreira <60441552+JoaoFerreira-FrontEnd@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/checkbox/checkbox.ionic.scss

@@ -0,0 +1,137 @@
+@import "./checkbox";
+@import "./checkbox.ionic.vars";
+
+// Ionic Checkbox
+// --------------------------------------------------
+
+:host {
+  // Border
+  --border-radius: #{$checkbox-ionic-border-radius};
+  --border-width: #{$checkbox-ionic-border-width};
+  --border-style: #{$checkbox-ionic-border-style};
+  --border-color: #{$checkbox-ionic-background-color-off};
+  --checkmark-width: 3;
+
+  // Focus
+  --focus-ring-color: #9ec4fd;
+  --focus-ring-width: 2px;
+  --focus-ring-offset: 2px;
+  
+  // Size
+  --size: #{$checkbox-ionic-size};
+
+  // Checkbox Target area
+  // -------------------------------------------------- 
+  &::after {
+    @include position(50%, 0, null, 0);
+
+    position: absolute;
+
+    height: 100%;
+    min-height: 48px;
+
+    transform: translateY(-50%);
+
+    content: "";
+
+    cursor: pointer;
+
+    z-index: 1;
+  }
+
+  .native-wrapper {
+    position: relative;
+  }
+}
+
+// Ionic Design Checkbox Sizes
+// --------------------------------------------------
+:host(.checkbox-size-small) {
+   // Size
+   --size: #{$checkbox-ionic-small-size};
+}
+
+// Ionic Design Checkbox Invalid
+// --------------------------------------------------
+:host(.ion-invalid) {
+  --focus-ring-color: #ffafaf;
+
+  .checkbox-icon {
+    border-color: #f72c2c;
+  }
+}
+
+// Checkbox Disabled
+// --------------------------------------------------
+// disabled, indeterminate checkbox
+:host(.checkbox-disabled.checkbox-indeterminate) .checkbox-icon {
+  /* TODO(FW-6183): Use design token variables */
+  border-width: 0;
+
+  background-color: #{$background-color-step-600};
+}
+
+// disabled, unchecked checkbox
+:host(.checkbox-disabled) .checkbox-icon {
+  /* TODO(FW-6183): Use design token variables */
+  border-color: #c9c9c9;
+
+  background-color: #f5f5f5; // mix of #f5f5f5 with 60% #FFF
+}
+
+// disabled, checked checkbox
+:host(.checkbox-disabled.checkbox-checked) .checkbox-icon {
+  border-width: 0;
+
+  background-color: $background-color-step-100;
+}
+
+// Checkbox Hover
+// --------------------------------------------------------
+@media (any-hover: hover) {
+  :host(:hover) .checkbox-icon {
+    /* TODO(FW-6183): Use design token variables */
+    background-color: #ececec; // mix of 'white', '#121212', 0.08, 'rgb'
+  }
+
+  :host(:hover.checkbox-checked) .checkbox-icon,
+  :host(:hover.checkbox-indeterminate) .checkbox-icon {
+    /* TODO(FW-6183): Use design token variables */
+    background-color: #1061da; // mix of '#1068eb', '#121212', 0.08, 'rgb'
+  }
+}
+
+// Checkbox Focus
+// --------------------------------------------------
+// Only show the focus ring when the checkbox is focused and not disabled
+:host(.ion-focused:not(.checkbox-disabled)) .checkbox-icon {
+  outline: var(--focus-ring-width) solid var(--focus-ring-color);
+  outline-offset: var(--focus-ring-offset);
+}
+
+// Checkbox: Active
+// --------------------------------------------------------
+:host(.ion-activated) .checkbox-icon {
+  /* TODO(FW-6183): Use design token variables */
+  background-color: #e3e3e3; // mix of 'white', '#121212', 0.12, 'rgb'
+}
+
+:host(.ion-activated.checkbox-checked) .checkbox-icon,
+:host(.ion-activated.checkbox-indeterminate) .checkbox-icon {
+  /* TODO(FW-6183): Use design token variables */
+  background-color: #105ed1; // mix of '#1068eb', '#121212', 0.12, 'rgb'
+}
+
+// Ionic Design Checkbox Shapes
+// --------------------------------------------------
+:host(.checkbox-shape-soft) {
+  --border-radius: #{$checkbox-ionic-border-radius};
+}
+
+:host(.checkbox-shape-rectangular) {
+  --border-radius: #{$checkbox-ionic-rectangular-border};
+}
+
+.checkbox-wrapper {
+  min-height: 48px;
+}