Remove theme-aware colors and prevent horizontal scroll on InterstitialModal

[mpshopify]

WHY are these changes introduced?

Stars and text on interstitial modal used theme-aware CSS colors, resulting in incorrect rendering on dark mode. It was also possible to horizontal scroll on the modal

WHAT is this pull request doing?

Replaces theme-aware CSS colors with hardcoded colors for the interstitial modal only. Prevents horizontal scroll on interstitial modal.

[shopify-shipitnext]

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