types/theme.nix: add check regex matching

Author: bloxx12

lib/types/theme.nix

@@ -1,13 +1,14 @@
 {lib}: let
-  inherit (lib.strings) isString hasPrefix;
+  inherit (lib.strings) isString;
   inherit (lib.types) mkOptionType;
-  inherit (builtins) stringLength;
+  inherit (builtins) match;
   # This was almost entirely taken from raf himself.
 in {
   hexColorType = mkOptionType {
     name = "hex-color";
     descriptionClass = "noun";
     description = "RGB color in hex format";
-    check = x: isString x && hasPrefix "#" x && stringLength x == 7;
+    # Check to determine wether the provided color is base16-valid
+    check = x: isString x && match "#[0-9a-fA-F]{6}" x;
   };
 }