Simplify retrieval of pygments_dark_style value

[AA-Turner]

I'm fairly sure that this was fixed by sphinx-doc/sphinx#10079, included in Sphinx 4.4 -- this theme now requires Sphinx 6.

[pradyunsg]

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sorted(vars(config))=['_raw_config', 'epub_css_files', 'extensions', 'html_css_files', 'html_js_files', 'html_theme', 'numfig_format', 'overrides', 'sample_theme_configuration_value', 'setup', 'source_suffix', 'values']
config.sample_theme_configuration_value='from conf.py'
config._raw_config['sample_theme_configuration_value']='from conf.py'
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sorted(vars(config))=['_raw_config', 'epub_css_files', 'extensions', 'html_css_files', 'html_js_files', 'html_theme', 'numfig_format', 'overrides', 'sample_theme_configuration_value', 'setup', 'source_suffix', 'values']
config.sample_theme_configuration_value='from conf.py'
config._raw_config['sample_theme_configuration_value']='from conf.py'
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Confirmed that things work well starting Sphinx v4.5.0.