Update zrep script with new color scheme and theme loading functionality.

- Added new base_colors associative array for color definitions.
- Implemented zrep_load_theme function to load themes.
- Modified zrep_msg function to use colors from the current theme.
- Updated zrep_version function to use theme colors for help and info.
- Adjusted zrep_list_installed_packages to use base_colors for color definitions.
- Enhanced zrep_update_package with informative messages.
- Added zrep_download_package function for downloading packages.
- Improved zrep_install_package to use zrep_download_package for downloads.
- Updated main function to load the global theme.

themes/classic

@@ -0,0 +1,10 @@
+# Classic zrep theme
+declare -A theme_colors=(
+    [std]="green"
+    [info]="yellow"
+    [debug]="red"
+    [other]="bold_yellow"
+    [sub]="magenta"
+    [main]="white green_bg" # Combining colors for 'main'
+    [help]="bold_green"
+)