I think it's possible as mini-player has already rounded corners.

In true I don't remember exactly how the shape is defined in mini-player, but will ask Petr to reply the details here.

To be honest, I'm not sure if it's possible. You can experiment with different CSS styles on a debug build by clicking Inspect Element after right clicking, allowing you to mess with different styles. Applying a border-radius style to any of the top-level elements (html, the document body, .winborder class) don't seem to have any effect. It might be a limitation of Chromium.

Wondering if anyone knows how to make the corners of the main window more rounded?