tui: color fixes

[chriso]

This PR fixes a few issues with colors:

• we avoid using white, in case the user has a light/white background, instead using lipgloss.NoColor which is presumably white or black depending on the theme/background
• we use ANSI(256) colors now, and use a slightly darker shade of green/yellow/red that looks better on light and dark background
• we use lipgloss to style the log prefixes, and avoid manually parsing color related environment variables (which lipgloss does for us)

This fixes #31.