Responsive site with dark-light mode toggle using Vanilla CSS and JS

• I was unable to implement a border radius with a gradient border on the Instagram Card. I would appreciate it if someone could provide some good resources for this.
• Also, I'm a newbie at JS. I would love to get some insights about the best practices to write JS code because I think the light-to-dark mode logic could be written more efficiently.
• I'm open to any other suggestions as well. Feel free to comment about it.

Thank You