Hi,
Per my understanding, you might want to change the background color and size of OOTB modal dialog.
When inspecting the HTML source of the modal dialog, we can see that the related CSS classes for the OOTB Modal Dialog are as below:

Thus, a possible solution is that you can write custom CSS style to override these built-in classes to implement your own style.
More information about how to add custom CSS style in SharePoint:
http://techtrainingnotes.blogspot.com/2012/05/adding-javascript-and-css-to-sharepoint.html
About how to write custom CSS style:
http://www.w3schools.com/css/
Thanks
Patrick Liang
TechNet Community Support
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