Allow transparent as a valid value for background-color

Problem
The Artisan module currently only allows background colors to be defined using standard color formats (HEX, RGB, HSL).
However, it does not allow background-color: transparent, which is a useful option when creating overlays, transparent sections, or custom UI elements.

Solution Implemented

• Updated getDefaultDefinition() in ArtisanCustomizations.php to accept "transparent" as a valid background color value.
• Modified getAttachmentStyles() to handle transparent
• Ensured that transparent can be saved in the UI and reflected in the generated CSS.

Testing Conducted
Transparent background setting

• Tested entering "transparent" in the background-color field within the UI.
• Verified that --theme-background-color: transparent; was generated correctly in the CSS.
• Ensured the transparent background rendered correctly on the frontend without affecting other styles.