themes/nanotech: Use the new built-in zsh color variables.

themes/nanotech.zsh-theme

-PROMPT=' %{$fg_bold[green]%}%2c %{$fg_bold[blue]%}%{[%}%{$reset_color%} '
-RPROMPT='$(git_prompt_info) %{$fg[blue]%}] %{$fg[green]%}%D{%L:%M} %{$fg[yellow]%}%D{%p}%{$reset_color%} '
+PROMPT='%F{green}%2c%F{blue} [%f '
+RPROMPT='$(git_prompt_info) %F{blue}] %F{green}%D{%L:%M} %F{yellow}%D{%p}%f'
 
-ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[yellow]%}"
-ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%}"
-ZSH_THEME_GIT_PROMPT_DIRTY=" %{$fg[red]%}*%{$reset_color%}"
+ZSH_THEME_GIT_PROMPT_PREFIX="%F{yellow}"
+ZSH_THEME_GIT_PROMPT_SUFFIX="%f"
+ZSH_THEME_GIT_PROMPT_DIRTY=" %F{red}*%f"
 ZSH_THEME_GIT_PROMPT_CLEAN=""