Button: fix styling of buttons with on-background within Vue3 apps

[michael-iden]

This change: (check at least one)

• [ ] Adds a new feature
• [X] Fixes a bug
• [ ] Improves maintainability
• [ ] Improves documentation
• [ ] Is a release activity

Is this a breaking change? (check one)

• [X] Yes
• [ ] No

Is the: (complete all)

• [X] Title of this pull request clear, concise, and indicative of the issue number it addresses, if any?
• [ ] Test suite(s) passing?
• [ ] Code coverage maximal?
• [ ] Changeset added?

What does this change address? Fixes #307

How does this change work? Changes the on-background to is-on-background

Additional context Yalc'd this change over the the Vue3 app and confirmed the button displays correctly when prefixed with is-

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: bed8ef36537efe028b2151903158f202c5201fb1

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[github-actions[bot]]

size-limit report 📦

Path	Size
packages/pharos/lib/index.js	47.94 KB (+0.2% 🔺)

[daneah]

This is only one of a few components that have an on-background attribute, by the looks of it. I would suggest we can eventually merge this pull request without changing those other components, but we might want to broaden #307 to all relevant components and remove Fixes #307 from this PR so that it doesn't get erroneously closed.

[daneah]

Subsumed by #578, #657, and possibly others.