Hard to say. Both the folder and the file are completely dependent upon which SP theme you are using and whether you have created a custom or child theme. Only you will know that. And dependent upon this it might be a .php or a ,css file. Again we do not know.
When you find it though – there should be a rule for .ui-dialog already in place for you to edit..
well as you already have complete admin access to the site, the control panel, wp , sp and remote ftp access through filezilla, you should already know what theme is being used, therefore should be a easy fix for you to do.If you don’t :
once logged in go to dashboard>appearances>themes>templates
for the control panel and/or ftp access to do php or css coding refer to the pm you got for access.
creating a custom theme or child theme is a customization beyond the scope of the support.. We can only tell you what the issue is with your site – in this case a greedy wp theme…
I did go ahead and add the custom css you were requested to add to your wp theme customizations… and looks fine now..