Update API styling for changes in sphinx 4.x

[jorisvandenbossche]

This is specifically updating the CSS for C++ API docs. Those got some additional styling in recent sphinx versions (https://github.com/sphinx-doc/sphinx/pull/9023/files#diff-25843cc831c4c93b5a56abd6c46e8964a68ce9cabfdf7b538fdc3dc37d82608fR546-R566), which gave an even bigger mixture of colors (with a blue that we don't use anywhere else):

This PR is overriding that added CSS to use our base color variables.

In addition, this PR also gives a color to the main function/method/attribute name in the signature (sig-name class). This is related to the changes in sphinx we discussed in #460. This doesn't fully tackle that issue (I also think that the single font size for example is an improvement). But using a different color for the main function name can be useful to quickly see which functions are documented in an overview (see eg https://arrow.apache.org/docs/python/generated/pyarrow.Date64Array.html#pyarrow.Date64Array, if you scroll down to under the table)

[choldgraf]

This all looks reasonable to me! One quick question: if people use Sphinx 3, will this make things look weird for them? If not then I think we should just merge and release 🚀

[jorisvandenbossche]

Good question: I now built our demo docs with sphinx 3.5.4, and that looks fine (some of the class names just don't exist yet, and eg the sig-name color I added was already like that before because it took the color of code divs.