Centering sidebars popup window

Hello. I’m trying to center popup window. I don’t know why it isn’t centering it by default. For reference i included 2 attachments. https://demo.vaadin.com/valo-theme/#!common
My popup is always aligned more to the left. I tried adding css rule for element and set left: 0px but it wasnt working. See attachments. So any ideas on how to fix it?
Thanks in advance.

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