apply default background css for icon (#842)

Author: DRSDavidSoft

lib/core/show-dir/styles.js

@@ -4,7 +4,7 @@ const icons = require('./icons.json');
 
 const IMG_SIZE = 16;
 
-let css = `i.icon { display: block; height: ${IMG_SIZE}px; width: ${IMG_SIZE}px; }\n`;
+let css = `i.icon { display: block; height: ${IMG_SIZE}px; width: ${IMG_SIZE}px; background: no-repeat center; }\n`;
 css += 'table tr { white-space: nowrap; }\n';
 css += 'td.perms {}\n';
 css += 'td.file-size { text-align: right; padding-left: 1em; }\n';