Update Sphinx theme #6728

Job	Run time
Checks (ubuntu-latest, 3.8)	20m 15s
Nature (ubuntu-latest, 3.8)	6m 32s
Tutorials (ubuntu-latest, 3.8)	38m 52s
Tutorials (ubuntu-latest, 3.11)	16m 59s
Nature (ubuntu-latest, 3.9)	6m 58s
Nature (ubuntu-latest, 3.10)	8m 37s
Nature (ubuntu-latest, 3.11)	4m 58s
Nature (macos-latest, 3.8)	8m 18s
Nature (macos-latest, 3.11)	7m 10s
Nature (windows-2019, 3.8)	27m 32s
Nature (windows-2019, 3.11)	10m 25s
Deprecation_Messages_and_Coverage (3.8)	21s
	2h 36m 57s

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