zsh: set autosuggestion color

Before, one could only toggle autosuggestion. This allows to set the
style as well.

modules/programs/zsh.nix

@@ -252,6 +252,7 @@ in
 
 {
   imports = [
+    (mkRenamedOptionModule [ "programs" "zsh" "enableAutosuggestions" ] [ "programs" "zsh" "autosuggestion" "enable" ])
     (mkRenamedOptionModule [ "programs" "zsh" "enableSyntaxHighlighting" ] [ "programs" "zsh" "syntaxHighlighting" "enable" ])
     (mkRenamedOptionModule [ "programs" "zsh" "zproof" ] [ "programs" "zsh" "zprof" ])
   ];
@@ -352,11 +353,6 @@ in
         type = types.lines;
       };
 
-      enableAutosuggestions = mkOption {
-        default = false;
-        description = "Enable zsh autosuggestions";
-      };
-
       zprof.enable = mkOption {
         default = false;
         description = ''
@@ -376,6 +372,24 @@ in
         description = "Options related to zsh-history-substring-search.";
       };
 
+      autosuggestion = {
+        enable = mkOption {
+          type = types.bool;
+          default = false;
+          description = "Enable zsh autosuggestions";
+        };
+
+        highlight = mkOption {
+          type = types.nullOr types.str;
+          default = null;
+          example = "fg=#ff00ff,bg=cyan,bold,underline";
+          description = ''
+            Custom styles for autosuggestion highlighting. See
+            {manpage}`zshzle(1)` for syntax.
+          '';
+        };
+      };
+
       history = mkOption {
         type = historyModule;
         default = {};
@@ -582,13 +596,16 @@ in
         # as all $fpath entries will be traversed again.
         ${optionalString (cfg.enableCompletion && !cfg.oh-my-zsh.enable && !cfg.prezto.enable)
           cfg.completionInit
-        }
+        }''
 
-        ${optionalString cfg.enableAutosuggestions
-          "source ${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions/zsh-autosuggestions.zsh"
-        }
+        (optionalString cfg.autosuggestion.enable ''
+          source ${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions/zsh-autosuggestions.zsh
+        '')
+        (optionalString (cfg.autosuggestion.enable && cfg.autosuggestion.highlight != null) ''
+          AUTOSUGGESTION_HIGHLIGHT_COLOR="${cfg.autosuggestion.highlight}"
+        '')
 
-        ${optionalString cfg.oh-my-zsh.enable ''
+        (optionalString cfg.oh-my-zsh.enable ''
             # oh-my-zsh extra settings for plugins
             ${cfg.oh-my-zsh.extraConfig}
             # oh-my-zsh configuration generated by NixOS
@@ -602,8 +619,9 @@ in
               "ZSH_THEME=\"${cfg.oh-my-zsh.theme}\""
             }
             source $ZSH/oh-my-zsh.sh
-        ''}
+        '')
 
+        ''
         ${optionalString cfg.prezto.enable
             (builtins.readFile "${pkgs.zsh-prezto}/share/zsh-prezto/runcoms/zshrc")}