If it is not working as expected then my CSS code will likely be ignored. Please work with the theme developer to find a solution for this problem. From looking closer at the demo and then looking back at your site I believe the problem might actually be that the demo uses a logo that is smaller than the image you are using. Therefore, while CSS will fix the problem it is not the correct fix. I would suggest resizing the image using an image editor and uploading it again in the theme settings.
I hope this helps. Please tag the thread as resolved once you find a solution.
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