Ghostty validate theme

[HeitorAugustoLN]

I think it would be nice to have a ghostty +validate-theme <theme> for custom themes under $XDG_CONFIG_HOME/ghostty/themes. Just like there is ghostty +validate-config to validate the user configuration.

[jcollie]

If your custom theme is part of your config then +validate-config should validate it as well. +list-themes should print out any errors when trying to load the theme as well.