Properly fallback to available themes

[envex]

https://github.com/Shopify/polaris-viz/blob/main/packages/polaris-viz-core/src/hooks/useTheme.ts#L13C3-L13C80

In this case, the passedTheme is Uplift so we never fallback to Light in any of the other variables. Maybe this is an unexpected case - like if you're specifically passing in a theme and we don't have it, we throw. Whereas the other case would be you don't pass in a theme at all and we default to Light