fix(toggle): set switch icon color correctly (#28812)

Issue number: None

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## What is the current behavior?
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Similar to #28809, in my work for FW-5822, I found that I had unexpected
screenshot diffs. `--ion-color-dark` was not defined in our test
infrastructure, and this toggle icon was therefore displaying in the
wrong color in the tests.

## What is the new behavior?
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this PR. -->

- Toggle has the correct icon color even when the --ion-color-dark token
is not defined

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

Introduces style changes if the token is undefined, but it should always
be defined in real-world uses.

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## Other information

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screenshots of how the component looks before and after the change. -->

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Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/toggle/toggle.ios.scss

@@ -87,7 +87,7 @@
 .toggle-switch-icon {
   position: absolute;
 
-  color: var(--ion-color-dark);
+  color: #{ion-color(dark, base)};
 }
 
 :host(.toggle-ltr) .toggle-switch-icon {