fix(breadcumb): set background when focused on md (#27274)

Issue number: resolves #27273

---------

<!-- Please refer to our contributing documentation for any questions on
submitting a pull request, or let us know here if you need any help:
https://ionicframework.com/docs/building/contributing -->

<!-- Some docs updates need to be made in the `ionic-docs` repo, in a
separate PR. See
https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#modifying-documentation
for details. -->

<!-- Please do not submit updates to dependencies unless it fixes an
issue. -->

<!-- Please try to limit your pull request to one type (bugfix, feature,
etc). Submit multiple pull requests if needed. -->

## What is the current behavior?
When breadcrumb is focused in md mode, background is not set.

## What is the new behavior?
A slight background should have applied on breadcrumb when focused.


## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!-- If this introduces a breaking change, please describe the impact
and migration path for existing applications below. -->


## Other information

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

core/src/components/breadcrumb/breadcrumb.md.scss

@@ -9,7 +9,7 @@
   --color-active: #{$breadcrumb-md-color-active};
   --color-hover: #{$breadcrumb-md-color-active};
   --color-focused: #{$breadcrumb-md-color-focused};
-  --background-focused: $breadcrumb-md-background-focused;
+  --background-focused: #{$breadcrumb-md-background-focused};
 }
 
 :host(.breadcrumb-active) {