Support dark mode

## Description

The UI should allow switching to dark mode and offer the option to follow the system theme.

## Additional Context

The colors used throughout the app have been reworked and consolidated. This makes it a bit simpler to roll out support for dark mode using Tailwind.

The prototype branch below is a good starting point. It still needs some minor tweaks and a color mode selector.

## Prototype branch

https://github.com/kmcginnes/graph-explorer/tree/feature/colors/dark-mode

## Prototype Screenshot

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