Fix logo in README when dark mode is on #6868

Job	Run time
build-windows (3.8)	5m 59s
build-ubuntu (3.8, Repository only)	1m 27s
build-ubuntu (3.8, Everything else)	3m 52s
build-ubuntu (3.13, Repository only)	2m 1s
build-ubuntu (3.13, Everything else)	4m 25s
build-ubuntu (3.13, lfs)	57s
build-ubuntu (3.8, fastai)	1m 21s
build-ubuntu (3.10, fastai)	56s
build-ubuntu (3.8, tensorflow)	1m 5s
build-ubuntu (3.10, tensorflow)	59s
build-ubuntu (3.8, Python 3.8, torch_1.11)	1m 5s
build-ubuntu (3.12, torch_latest)	1m 0s
	25m 7s

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