feat: add theme field to Icon type for light/dark mode support #438

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	37s
checks / test (3.11, lowest-direct, windows-latest)	2m 55s
checks / test (3.10, lowest-direct, windows-latest)	3m 41s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.11, locked, windows-latest)	2m 30s
checks / test (3.12, lowest-direct, windows-latest)	3m 41s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 22s
checks / test (3.14, lowest-direct, windows-latest)	2m 30s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.11, lowest-direct, ubuntu-latest)	49s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.14, locked, windows-latest)	3m 32s
checks / test (3.10, lowest-direct, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, locked, windows-latest)	2m 32s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.14, locked, ubuntu-latest)	2m 17s
checks / test (3.10, locked, windows-latest)	2m 32s
checks / test (3.13, lowest-direct, windows-latest)	2m 57s
checks / test (3.12, locked, windows-latest)	3m 25s
all-green	4s
	41m 36s

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