add preview 'centering' snippet

bobheadxi · bobheadxi · commit ed17877b907a · 2021-07-20T08:28:13.000+08:00
diff --git a/preview-vertical.css b/preview-vertical.css
@@ -0,0 +1,4 @@
+.markdown-preview-sizer .markdown-preview-section {
+    padding-top: 20%;
+    padding-bottom: 25%;
+}
