Fix dark-theme in html[data-theme=dark]-tags

[prisae]

This is similar to #6500 and #6501 : In my docs, the dark-theme is noted in the html-part as

html< ... data-theme=dark ... >

and xarray-tables look therefore awful with the dark theme, as they are not rendered properly, as can be seen, e.g., here: https://emsig.xyz/emg3d-gallery/gallery/tutorials/simulation.html#create-survey.

This can be fixed by extending

xarray/xarray/static/css/style.css

Lines 16 to 18 in fff8253

	html[theme=dark],
	body[data-theme=dark],
	body.vscode-dark {

to

html[theme=dark],
html[data-theme=dark],
body[data-theme=dark],
body.vscode-dark {

• User visible changes (including notable bug fixes) are documented in whats-new.rst

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[dcherian]

Thanks!

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