A dedicated COPS (Calibre OPDS) css theme for kindle e-book readers.
Should also work elegantly on other manufacturers e-book readers.

I was struggling with unbrowse'able cops library using default COPS templates.
The bootstrap theme made things even worse, as everything is completely unreadable on kindle readers.
Since I haven't found in inetwebz any custom made template for COPS dedicated to ebook readers I had to roll up my own sleeves.

Just copy the css file to /templates/default/styles/ directory within your COPS installation.
The theme will be available to choose on cops configuration page right away.

I could test this only on kindle voyage (gen.7) having 300ppi display.
Any newer kindle generation shouldn't have problems with diplaying nicely ebook lists, thumbnails, descriptions and download links.