fix (shadow color): support custom properties

[bfintal]

fixes #3176

[github-actions]

🤖 Pull request artifacts

file	commit
pr3242-stackable-3242-merge.zip	dc1be92

[github-actions]

Try this Pull Request in the WordPress playground: https://playground.wordpress.net/?mode=seamless#{"landingPage":"/wp-admin/post-new.php?post_type=page","preferredVersions":{"php":"latest","wp":"latest"},"steps":[{"step":"login","username":"admin","password":"password"},{"step":"installPlugin","pluginZipFile":{"resource":"url","url":"https://github.com/raw/gambitph/Stackable/artifacts/pr3242-stackable-3242-merge.zip"}}]}

[andeng1106]

@bfintal Issues found: https://www.notion.so/3176-issues-b92b4113901c43dab8d89b9057b2dbc8