Define and use custom Chakra theme

[whoabuddy]

The project started with some bare-bones CSS, and has evolved quite a bit since.

There are some hacks to get things the way they are, primarily in index.css.

It looks fine, but certain things don't work or work differently than expected, like applying a variant to a button outline.

Sometime after that, a custom theme was added to make the page start/stay in dark mode in bb2a09c.

This could be used to properly override the various hacks and would be worth exploring for more consistency.

Ref: https://chakra-ui.com/docs/styled-system/customize-theme