Featured Image size changes

Hi,

I’m having trouble keeping my featured image the same width as my content. Featured images are the same width in the Customizer but a bit wider in preview.

I’ve added the snippet to control the featured image size in the Custom CSS field:

.single .thumbnail img {
max-width: 72%;
margin: 0 auto;
display: flex;
}

I set “max-width” to the same percentage as General Settings > content width (72%). Layout is “full width”.

Blog > single post layout is also “full width” with max width at 700.

I wonder if “display: flex” has something to do with the change?

Thanks in advance.