Because

This PR enhances the "Custom properties" lesson by adding an additional resource, helping learners better understand practical applications of the topic.

This PR

• Added a new resource link to the "Custom properties" section.
• Updated lesson content with a brief explanation of the resource.

Issue

Closes #29243

Additional Information

No additional information.

Pull Request Requirements

• [ x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [ x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• [ x] The Because section summarizes the reason for this PR
• [ x] The This PR section has a bullet point list describing the changes in this PR
• [ x] If this PR addresses an open issue, it is linked in the Issue section
• [ x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [ x] If any lesson files are included in this PR, they follow the Layout Style Guide