make appearance of lightbox configurable

assets/scss/snap-gallery.scss

@@ -10,7 +10,7 @@ $gap: var(--gap);
 
   /* Create equal columns in flexbox */
   .snap-image {
-    cursor: zoom-in;
+    cursor: var(--cursor);
 
     /* Column amount and width are configurable by style variables */
     width: calc(100% / var(--columns) - #{$gap});
@@ -31,7 +31,8 @@ $gap: var(--gap);
 
 .snap-slideshow {
   width: var(--slideshow-width);
-  margin: auto;
+  margin-left: auto;
+  margin-right: auto;
 
   // Hide all contained images except the first
   .snap-image:not(:first-child) {