Covered Theme module with tests

- Extracted `ColorReplacementGenerator`;
- Extracted `ColorConverterImpl`;
- Added documentation to some classes;
- `hex_to_rgba` now supports shorthand hex colors (#fff).

scripts/utils/style_manager.py

@@ -1,19 +1,40 @@
+import os
+
 from scripts.utils import generate_file
 
 
 class StyleManager:
+    """Manages the style files for the theme."""
+
     def __init__(self, output_file: str):
+        """
+        :param output_file: The path to the output file where styles will be combined.
+        """
         self.output_file = output_file
 
     def append_content(self, content: str):
+        """
+        Append content to the output file.
+        :raises FileNotFoundError: if the file does not exist.
+        """
+        if not os.path.exists(self.output_file):
+            raise FileNotFoundError(f"The file {self.output_file} does not exist.")
         with open(self.output_file, 'a') as output:
-            output.write(content + '\n')
+            output.write('\n' + content)
 
     def prepend_content(self, content: str):
+        """
+        Prepend content to the output file.
+        :raises FileNotFoundError: if the file does not exist.
+        """
         with open(self.output_file, 'r') as output:
             main_content = output.read()
         with open(self.output_file, 'w') as output:
             output.write(content + '\n' + main_content)
 
     def generate_combined_styles(self, sources_location: str, temp_folder: str):
+        """
+        Generate the combined styles file
+        by merging all styles from the source location.
+        """
         generate_file(sources_location, temp_folder, self.output_file)