Default theme uses singleton instance of `themeOptions`

[chanceaclark]

Describe the bug @bigcommerce/big-design-theme uses singleton instance of themeOptions. This can cause issues if we want to create two separate themes with different font-sizes.

To Reproduce Codesandbox to replicate the issue: https://codesandbox.io/s/delicate-thunder-7ih0r?file=/src/index.tsx

The first box in the codesandbox is being remCalc'd to 48px but you can see the calculation is using the 14px font-size for the calculation.

Expected behavior Each theme should have it's own instance of themeOptions to ensure font-sizes and potential future options don't clash with another themes.

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