diff --git a/rendercv/data_models.py b/rendercv/data_models.py
index 7be0e86..73585aa 100644
--- a/rendercv/data_models.py
+++ b/rendercv/data_models.py
@@ -936,6 +936,8 @@ RenderCVDesign = Annotated[
     ClassicThemeOptions | ModerncvThemeOptions | McdowellThemeOptions,
     pydantic.Field(discriminator="theme"),
 ]
+rendercv_design_validator = pydantic.TypeAdapter(RenderCVDesign)
+available_themes = ["classic", "moderncv", "mcdowell"]
 
 
 class RenderCVDataModel(RenderCVBaseModel):
@@ -951,20 +953,16 @@ class RenderCVDataModel(RenderCVBaseModel):
         description="The design information of the CV.",
     )
 
-    @pydantic.field_validator("design")
+    @pydantic.field_validator("design", mode="before")
     @classmethod
     def initialize_if_custom_theme_is_used(
         cls, design: RenderCVDesign | Any
     ) -> RenderCVDesign | Any:
         """Initialize the custom theme if it is used and validate it. Otherwise, return
         the built-in theme."""
-        # `get_args` for an Annotated object returns the arguments when Annotated is
-        # used. The first argument is actually the union of the types, so we need to
-        # access the first argument to use isinstance function.
-        theme_data_model_types = get_args(RenderCVDesign)[0]
-        if isinstance(design, theme_data_model_types):
-            # it is a built-in theme
-            return design
+        if design["theme"] in available_themes:  # type: ignore
+            # it is a built-in theme, validate and return it:
+            return rendercv_design_validator.validate_python(design)
         else:
             theme_name: str = design["theme"]  # type: ignore
             # check if the theme name is valid:
@@ -1061,14 +1059,21 @@ def read_input_file(
     """
     # check if the file exists:
     if not file_path.exists():
-        raise FileNotFoundError(f"The input file {file_path} doesn't exist.")
+        raise FileNotFoundError(
+            f"The input file [magenta]{file_path}[/magenta] doesn't exist!"
+        )
 
     # check the file extension:
     accepted_extensions = [".yaml", ".yml", ".json", ".json5"]
     if file_path.suffix not in accepted_extensions:
+        user_friendly_accepted_extensions = [
+            f"[green]{ext}[/green]" for ext in accepted_extensions
+        ]
+        user_friendly_accepted_extensions = ", ".join(user_friendly_accepted_extensions)
         raise ValueError(
             "The input file should have one of the following extensions:"
-            f" {accepted_extensions}. The input file is {file_path}."
+            f" {user_friendly_accepted_extensions}. The input file is"
+            f" [magenta]{file_path}[/magenta]."
         )
 
     file_content = file_path.read_text(encoding="utf-8")