fixed detailed-solution bug

common/lib/xmodule/xmodule/css/capa/display.scss

@@ -67,18 +67,20 @@ section.problem {
     }
   }
 
-  span.solution-span {
-    margin: 20px 0;
-    display: block;
-    border: 1px solid #ddd;
-    padding: 9px 15px 20px;
-    background: #FFF;
-    position: relative;
-    @include box-shadow(inset 0 0 0 1px #eee);
-    @include border-radius(3px);
+  .solution-span {
+    > span {
+      margin: 20px 0;
+      display: block;
+      border: 1px solid #ddd;
+      padding: 9px 15px 20px;
+      background: #FFF;
+      position: relative;
+      @include box-shadow(inset 0 0 0 1px #eee);
+      @include border-radius(3px);
 
-    &:empty {
-      display: none;
+      &:empty {
+        display: none;
+      }
     }
   }
 
@@ -478,14 +480,6 @@ section.problem {
   }
 
   .detailed-solution {
-    border: 1px solid #ddd;
-    padding: 9px 15px 20px;
-    margin: 20px 10px;
-    background: #FFF;
-    position: relative;
-    @include box-shadow(inset 0 0 0 1px #eee);
-    @include border-radius(3px);
-
     > p:first-child {
       font-size: 0.9em;
       font-weight: bold;