Add support for fotorama 'thumbmargin' setting within a theme's view.xml

<vars module="Magento_Catalog">
    <var name="gallery">
        <var name="thumbmargin">16</var>
    </var>
</var>

app/code/Magento/Catalog/view/frontend/templates/product/view/gallery.phtml

@@ -68,6 +68,9 @@
                         "thumbheight": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_small', 'height')
                         ?: $block->getImageAttribute('product_page_image_small', 'width'); ?>,
                     <?php endif; ?>
+                    <?php if (($block->getVar("gallery/thumbmargin"))): ?>
+                        "thumbmargin": <?php echo $block->getVar("gallery/thumbmargin"); ?>,
+                    <?php endif; ?>
                     <?php if ($block->getImageAttribute('product_page_image_medium', 'height') || $block->getImageAttribute('product_page_image_medium', 'width')): ?>
                         "height": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_medium', 'height')
                         ?: $block->getImageAttribute('product_page_image_medium', 'width'); ?>,