[Bug]: sp-theme applies Adobe font-family to non-Spectrum child elements

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-theme

### Expected behavior

The sp-theme element should only apply font-family to encapsulated Spectrum web components, and not to other elements contained in its light DOM.

### Actual behavior

The sp-theme element applies system font-family styles to all encapsulated elements through a :host CSS rule applied to its shadow root.

### Screenshots


### What browsers are you seeing the problem in?

Firefox, Chrome

### How can we reproduce this issue?

1. Wrap a 'div' or other element in an sp-theme
2. Look at element
3. Element has Spectrum font-family applied

### Sample code or abstract reproduction which illustrates the problem

```

```

### Severity

SEV 4

### Logs taken while reproducing problem

_No response_

Locate the sp-theme component entry point and its shadow-root styles; reproduce the sample with a non-Spectrum light-DOM div and inspect the applied font-family. Check the component's existing tests, then confirm the fix preserves font styling for Spectrum children while leaving the div unstyled.

Written by the indexing model from the issue text.