Custom split view border color and a small tweak to the split view behavior

Hi, I absolutely enjoy the new Split View feature but I think it would be even greater if we get an option to change the color of the border itself. My reasoning for this is that it adds just that small bit of customization to make our Brave setups our own.

And speaking of Split View, when I close the right-most tab in a split view, it switches to the tab immediately after it. In my opinion, the expected behavior is that it should switch to the tab it was in a split with since they were essentially in a “group”.