Add webkit-transform for proper Safari centering

Fixes centering the rs-popover-message in Safari.
Tabs instead of spaces!

![](https://camo.githubusercontent.com/7e6fd44e8883f7f2af48f585646e355e448cd6f6/687474703a2f2f69313238382e70686f746f6275636b65742e636f6d2f616c62756d732f623438332f70726f66616e6f6e74756d626c722f72494e4a6a5f7a707366353839376362392e676966)