[#97] Refactor default themes

[aome510]

Resolves #97

TODO

• remove built-in themes in favour of a single default ANSI theme
• update theme_parse script
• update theme/palette documentation
• update theme.toml and app.toml examples

Theme config changes

• made background and foreground fields optional in a theme's palette. Default to use terminal's background/foreground colors if not specified.
• made 16 ANSI color fields optional in a theme's palette. Default to use terminal's ANSI colors if not specified.
• removed selection_background and selection_foreground fields from a theme's palette
• tweaked default value for playback_progress_bar component style