fix: Fix the debug classNames for stylex.createTheme

[nmn]

What changed / motivation ?

stylex.createTheme was almost always generating UnknownFile__* as debug classNames. Fixed it to use the actual file name, just like stylex.create.

[github-actions]

compressed-size: runtime library

Size change: 0.00 kB
Total size: 2.52 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./packages/stylex/lib/stylex.js	1.04 (3.22)	0.00 (0.00)	0.0% (0.0%)
./packages/stylex/lib/StyleXSheet.js	1.48 (3.75)	0.00 (0.00)	0.0% (0.0%)

[github-actions]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1128.55 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./apps/rollup-example/.build/bundle.js	1005.20 (10185.36)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	123.34 (773.82)	0.00 (0.00)	0.0% (0.0%)