Fix low contrast in dark theme h3 headings

[dstansby]

The contrast between text and background is bad for h3 elements:

This is because the colour is manually set in the zarr CSS - removing this fixes the low contrast by making the titles white.

[d-v-b]

the change seems reasonable, do we know why the manual adjustment was made?

[dstansby]

It came in this rather large commit: 4dc6f1f. I think it's worth relying on pydata-sphinx-theme to set this to reduce the amount of custom CSS we have to maintain.