Add more themes

[orhun]

Is your feature request related to a problem? Please describe.

The currently supported themes are listed in README.md and we need more themes!

Describe the solution you'd like

Adding a theme is easy:

1. Add the new variant to BuiltinTheme enum:

https://github.com/orhun/flawz/blob/32fb9ac656092404bad931c0e2e59438191ea0ab/src/theme.rs#L5-L17

2. Add a new function for the theme colors:

https://github.com/orhun/flawz/blob/32fb9ac656092404bad931c0e2e59438191ea0ab/src/theme.rs#L62-L89

3. Add screenshot in assets/ and update README.md

Describe alternatives you've considered

None.

Additional context

I would love to have a light theme.