Refactor GDM module to correspond SOLID principles

- I don't know how I will support this hell ahh code;
- Added some methods to gnome_shell_theme_builder.py;
- Added "color" property "install" method from theme_base.py.

scripts/utils/global_theme/ubuntu_alternatives_updater.py

@@ -0,0 +1,65 @@
+import os.path
+
+from scripts import config
+from scripts.utils.alternatives_updater import AlternativesUpdater
+
+
+class UbuntuGDMAlternativesUpdater:
+    """
+    Manages update-alternatives for Ubuntu GDM themes.
+
+    This class handles:
+    - Creating alternatives for GDM theme files
+    - Setting installed theme as the active alternative
+    - Removing theme alternatives during uninstallation
+    """
+    def __init__(self, alternatives_updater: AlternativesUpdater):
+        """
+        :param alternatives_updater: Handler for update-alternatives operations
+        """
+        self.ubuntu_gresource_link = config.ubuntu_gresource_link
+        self.destination_dir = config.global_gnome_shell_theme
+        self.destination_file = config.gnome_shell_gresource
+
+        self.alternatives_updater = alternatives_updater
+
+        self._update_gresource_paths()
+
+    def _update_gresource_paths(self):
+        self.ubuntu_gresource_path = os.path.join(self.destination_dir, self.ubuntu_gresource_link)
+        self.gnome_gresource_path = os.path.join(self.destination_dir, self.destination_file)
+
+    def with_custom_destination(self, destination_dir: str, destination_file: str):
+        """Set custom destination directory and file for the theme."""
+        self.destination_dir = destination_dir
+        self.destination_file = destination_file
+        self._update_gresource_paths()
+        return self
+
+    def install_and_set(self, priority: int = 0):
+        """
+        Add theme as an alternative and set it as active.
+
+        This creates a system alternative for the GDM theme and
+        makes it the active selection with the specified priority.
+
+        :param priority: Priority level for the alternative (higher wins in conflicts)
+        """
+        self.alternatives_updater.install_and_set(
+            link=self.ubuntu_gresource_path,
+            name=self.ubuntu_gresource_link,
+            path=self.gnome_gresource_path,
+            priority=priority
+        )
+
+    def remove(self):
+        """
+        Remove the theme alternative from the system.
+
+        This removes the previously installed alternative, allowing
+        the system to fall back to the default GDM theme.
+        """
+        self.alternatives_updater.remove(
+            name=self.ubuntu_gresource_link,
+            path=self.gnome_gresource_path
+        )