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

[prisae]

This is similar to https://github.com/pydata/xarray/issues/6500 and https://github.com/pydata/xarray/pull/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 https://github.com/pydata/xarray/blob/fff82539c7b0f045c35ace332c4f6ecb365a0612/xarray/static/css/style.css#L16-L18

to

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

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

[welcome[bot]]

Thank you for opening this pull request! It may take us a few days to respond here, so thank you for being patient. If you have questions, some answers may be found in our contributing guidelines.

[dcherian]

Thanks!

[welcome[bot]]

Congratulations on completing your first pull request! Welcome to Xarray! We are proud of you, and hope to see you again!