Fix types for theming with tests

[nmn]

Ran into some type errors when using the Theming APIs with Typescript. After much work, I managed to get it working correctly.

[github-actions]

compressed-size: runtime library

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./packages/stylex/lib/stylex.js	1.02 (3.53)	0.00 (0.00)	0.0% (0.0%)
./packages/stylex/lib/StyleXSheet.js	1.34 (3.41)	0.00 (0.00)	0.0% (0.0%)

[github-actions]

compressed-size: e2e bundles

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./apps/rollup-example/.build/bundle.js	1005.22 (9872.52)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	114.18 (455.95)	0.00 (0.00)	0.0% (0.0%)