Covered Theme module with tests

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

tests/utils/test_style_manager.py

@@ -0,0 +1,78 @@
+import os
+import shutil
+import unittest
+
+from scripts import config
+from scripts.utils.style_manager import StyleManager
+from .._helpers import create_dummy_file
+
+
+class StyleManagerTestCase(unittest.TestCase):
+    def setUp(self):
+        self.temp_folder = os.path.join(config.temp_tests_folder, "style_manager")
+        os.makedirs(self.temp_folder, exist_ok=True)
+        self.output_file = os.path.join(self.temp_folder, "output.css")
+        self.manager = StyleManager(output_file=self.output_file)
+
+    def tearDown(self):
+        shutil.rmtree(self.temp_folder, ignore_errors=True)
+
+    def test_append_content(self):
+        start_css = "body { background-color: blue; }"
+        create_dummy_file(self.output_file, start_css)
+        end_css = "h1 { color: red; }"
+
+        self.manager.append_content(end_css)
+
+        with open(self.output_file, 'r') as f:
+            content = f.read()
+            split_content = content.splitlines()
+            assert split_content[0] == start_css
+            assert split_content[1] == end_css
+        os.remove(self.output_file)
+
+    def test_append_does_not_create_file(self):
+        end_css = "h1 { color: red; }"
+
+        with self.assertRaises(FileNotFoundError):
+            self.manager.append_content(end_css)
+
+    def test_prepend_content(self):
+        start_css = "body { background-color: blue; }"
+        create_dummy_file(self.output_file, start_css)
+        prepend_css = "h1 { color: red; }"
+
+        self.manager.prepend_content(prepend_css)
+
+        with open(self.output_file, 'r') as f:
+            content = f.read()
+            split_content = content.splitlines()
+            assert split_content[0] == prepend_css
+            assert split_content[1] == start_css
+        os.remove(self.output_file)
+
+    def test_prepend_does_not_create_file(self):
+        prepend_css = "h1 { color: red; }"
+
+        with self.assertRaises(FileNotFoundError):
+            self.manager.prepend_content(prepend_css)
+
+    def test_generate_combined_styles(self):
+        source_folder = os.path.join(config.temp_tests_folder, "style_manager_source")
+        source_css_folder = os.path.join(source_folder, ".css")
+        first_file = os.path.join(source_css_folder, "file1.css")
+        second_file = os.path.join(source_css_folder, "file2.css")
+        first_css = "body { background-color: blue; }"
+        second_css = "h1 { color: red; }"
+        create_dummy_file(first_file, first_css)
+        create_dummy_file(second_file, second_css)
+
+        self.manager.generate_combined_styles(source_folder, self.temp_folder)
+
+        with open(self.output_file, 'r') as f:
+            content = f.read()
+            split_content = content.splitlines()
+            assert first_css in split_content
+            assert second_css in split_content
+        os.remove(self.output_file)
+        shutil.rmtree(source_folder, ignore_errors=True)