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.

Screenshots
N/A

Desktop (please complete the following information, if applicable):
N/A

Smartphone (please complete the following information, if applicable):
N/A

Additional context
N/A