[design] improve style guide

Author: zombieFox

src/css/form.css

@@ -1410,6 +1410,11 @@ input[type="range"]:disabled::-moz-range-progress{
 border-radius:var(--theme-radius);
 }
 
+.form-grid-1x3{
+grid-template-rows:repeat(3,1fr);
+border-radius:var(--theme-radius);
+}
+
 .form-grid .form-wrap{
 margin-top:0;
 margin-bottom:0;