Address review comments.

Use the more specific "background-color" instead of "background" where applicable.

drag_and_drop_v2/public/css/drag_and_drop.css

@@ -36,7 +36,7 @@
 .xblock--drag-and-drop .drag-container {
     width: auto;
     padding: 1%;
-    background: #ebf0f2;
+    background-color: #ebf0f2;
 }
 
 /*** DRAGGABLE ITEMS ***/
@@ -166,12 +166,12 @@
 }
 
 .xblock--drag-and-drop .drag-container .option .numerical-input.correct .input {
-    background: #ceffce;
+    background-color: #ceffce;
     color: #087108;
 }
 
 .xblock--drag-and-drop .drag-container .option .numerical-input.incorrect .input {
-    background: #ffcece;
+    background-color: #ffcece;
     color: #ad0d0d;
 }
 
@@ -190,7 +190,7 @@
     height: auto;
     position: relative;
     margin-top: 1%;
-    background: #fff;
+    background-color: #fff;
 }
 
 .xblock--drag-and-drop .target-img-wrapper {
@@ -274,7 +274,7 @@
     top: 5%;
     right: 5%;
     border: 1px solid #fff;
-    background: none repeat scroll 0 0 rgba(0, 0, 0, 0.8);
+    background-color: rgba(0, 0, 0, 0.8);
     width: 500px;
     max-width: 90%;
     min-height: 50px;
@@ -325,7 +325,7 @@
     left: 0;
     width: 100%;
     height: 100%;
-    background: #000;
+    background-color: #000;
     opacity: 0.5;
     z-index: 1500;
 }
@@ -349,7 +349,7 @@
 
 .xblock--drag-and-drop .modal-content {
     border-radius: 5px;
-    background: #ffffff;
+    background-color: #ffffff;
     margin-bottom: 5px;
     padding: 5px;
 }
@@ -379,6 +379,6 @@
     padding: 0;
     position: absolute;
     width: 1px;
-    background: #ffffff;
+    background-color: #ffffff;
     color: #000000;
 }

drag_and_drop_v2/public/css/drag_and_drop_edit.css

@@ -68,7 +68,7 @@
 
 .xblock--drag-and-drop--editor .tab {
     width: 100%;
-    background: #eee;
+    background-color: #eee;
     padding: 3px 0;
     position: relative;
 }
@@ -177,7 +177,7 @@
 }
 
 .xblock--drag-and-drop--editor .items-form .item {
-    background: #8fcaec;
+    background-color: #8fcaec;
     padding: 10px 0 1px;
     margin: 15px 0;
 }
@@ -219,7 +219,7 @@
 
 /** Buttons **/
 .xblock--drag-and-drop--editor .btn {
-    background: #1d5280;
+    background-color: #1d5280;
     color: #fff;
     border: 1px solid #156ab4;
     border-radius: 6px;
@@ -257,7 +257,7 @@
     width: 14px;
     height: 14px;
     border-radius: 7px;
-    background: #1d5280;
+    background-color: #1d5280;
     position: relative;
     float: left;
     margin: 0 5px 0 0;
@@ -274,7 +274,7 @@
     content: '';
     height: 10px;
     width: 2px;
-    background: #fff;
+    background-color: #fff;
     position: relative;
     display: inline;
     float: left;
@@ -286,7 +286,7 @@
     content: '';
     height: 2px;
     width: 10px;
-    background: #fff;
+    background-color: #fff;
     position: relative;
     display: inline;
     float: left;
@@ -298,7 +298,7 @@
     content: '';
     height: 10px;
     width: 2px;
-    background: #fff;
+    background-color: #fff;
     position: relative;
     display: inline;
     float: left;
@@ -313,7 +313,7 @@
     content: '';
     height: 2px;
     width: 10px;
-    background: #fff;
+    background-color: #fff;
     position: relative;
     display: inline;
     float: left;
@@ -325,10 +325,10 @@
 }
 
 .xblock--drag-and-drop--editor .remove-item .icon.remove {
-    background: #fff;
+    background-color: #fff;
     color: #0072a7;  /* Override default color from Studio to ensure contrast is large enough */
 }
 .xblock--drag-and-drop--editor .remove-item .icon.remove:before,
 .xblock--drag-and-drop--editor .remove-item .icon.remove:after {
-    background: #1d5280;
+    background-color: #1d5280;
 }

drag_and_drop_v2/public/themes/apros.css

 .themed-xblock.xblock--drag-and-drop {
-    background: #fff;
+    background-color: #fff;
 }
 
 /* Shared styles used in header and footer */
@@ -14,7 +14,7 @@
 /* drag-container holds the .item-bank and the .target */
 
 .themed-xblock.xblock--drag-and-drop .drag-container {
-    background: #ebf0f2;
+    background-color: #ebf0f2;
 }
 
 .themed-xblock.xblock--drag-and-drop .item-bank {
@@ -32,12 +32,12 @@
 }
 
 .themed-xblock.xblock--drag-and-drop .drag-container .option .numerical-input.correct .input {
-    background: #ceffce;
+    background-color: #ceffce;
     color: #087108;
 }
 
 .themed-xblock.xblock--drag-and-drop .drag-container .option .numerical-input.incorrect .input {
-    background: #ffcece;
+    background-color: #ffcece;
     color: #ad0d0d;
 }
 
@@ -48,7 +48,7 @@
 /*** DROP TARGET ***/
 
 .themed-xblock.xblock--drag-and-drop .target {
-    background: #fff;
+    background-color: #fff;
 }
 
 .themed-xblock.xblock--drag-and-drop .zone p {
@@ -66,7 +66,7 @@
 }
 
 .themed-xblock.xblock--drag-and-drop .popup {
-    background: none repeat scroll 0 0 #66a5b5;
+    background-color: #66a5b5;
 }
 
 .themed-xblock.xblock--drag-and-drop .popup .popup-content {