feature(zsh): only display a theme with glyphs if the terminal is able to render them

Reviewed-on: #26

modules/home-manager/shell.nix

@@ -1,15 +1,25 @@
 { config, pkgs, inputs, ... }:
  
 {
+  # configure z-shell to use omz with some plugins
   programs.zsh = {
     enable = true;
     autosuggestion.enable = true;
     syntaxHighlighting.enable = true;
     oh-my-zsh = {
       enable = true;
-      theme = "agnoster";
+      # theme will be set below
+      theme = "";
       plugins = [ "git" "ssh-agent" "gpg-agent" ];
     };
+    # only display a fancy theme when glyphs are rendered
+    initExtraBeforeCompInit = ''
+      if [[ "$TERM" == "linux" || "$TERM" == "screen" ]]; then
+        ZSH_THEME="gentoo"
+      else
+        ZSH_THEME="agnoster"
+      fi
+    '';
   };
 
   # set important env vars