How to customize GUI?

I know that you can use CSS to customize the reader but I'm wondering if there's anyway to customize the library management view?

Specifically I feel like everything is too tight and near the edge of the window. I'd like to add some padding to some elements. I searched around but couldn't find anything useful. Does this feature exist?


Also in the same vein but not worth creating another topic for: Can Calibre load two icon themes and switch between them on launch depending on light/dark mode (for MacOS)?