Extract theme/color configuration to separate module

[mrolli]

Is your feature request related to a current shortcoming? Please describe.
Currently, color scheme and theme-related configuration is scattered throughout wezterm.lua, making it harder to customize themes or switch color schemes.

Describe the solution you'd like
Extract theme-related configuration to a dedicated theme.lua or config/theme.lua module:

• Color scheme selection (currently line 42: "Gruvbox dark, soft (base16)")
• Tab bar colors (lines 56-71)
• Status bar colors (lines 119-132: colors_bg and colors_fg arrays)
• Powerline arrow symbols (could be part of theme)
• Font configuration (might be theme-related)

Additional context
This would make it easier to:

• Switch between different themes
• Maintain consistent colors across the configuration
• Share themes with others
• Keep the main config file focused on behavior rather than appearance