Improve dark mode

[manthey]

PR #344 added the ability to be in dark mode. This is done by color inversion and shifting for most of the UI with a few exceptions to avoid changing annotation and image colors. It could be done better with more appropriate css styles; the full method would be to upgrade the version of Bootstrap we use for styling, but there are probably other improvements.

If we have enough users think dark mode looks "good enough", we could expose a more obvious mode toggle somewhere rather than just have it configurable.