Added dark mode support to index.html

[danielwcarey]

Updated the index.html to default to system's selected light/dark mode selection and updated the banner to include toggle icons between light and dark. If the browser doesn't support theming, there should be no visible change, and the toggle icons won't appear.

I tried to make minimal edits but also provide something that may enable more themes if others are so inclined to pursue.

For reference, this is the snapshot of primary theme and then some screenshots of the dark theme
when applied to several pages:

[chrisgleissner]

Hi @danielwcarey ,

this is a great addition for anyone who is either sensitive to bright backgrounds (such as myself) or whoever wants a consistent look between U64 menu system and HTTP menu.

@GideonZ is currently working on 3.15 and merged a bunch of PRs into the test-merge branch in preparation for the release.

I see you have done extensive testing and also included screenshots, which is very useful. It may be worth to change the target branch to test-merge and also mention which device(s) you tested this on.

Thanks
Christian

[danielwcarey]

Squashed the two commits and rebased on the branch, "test-merge".

Confirmed on Edge browser, Version 150.0.4078.65. The update uses base CSS and JavaScript and should work on other browsers.