Configure Base16 colors for p10k

 - Adds and uses set of `BASE16_BASE*` variables in `p10k.zsh`
 - Note: Not all `FOREGROUND|BACKGROUND|COLOR` configs have been changed
   from default values, only the most important ones. There rest is left
   as a future work.
 - Also, note that this is a custom (manual) change to the p10k config
   and as such will be reverted by potential future `p10k configure`