feedback column header made theme aware

RetrospectiveExtension.Frontend/css/feedbackColumn.scss

@@ -1,4 +1,4 @@
-@import "_variables.scss";
+@import "_variables.scss";
 
 .feedback-column {
   border-right: 2px solid white;
@@ -17,7 +17,6 @@
 .feedback-column-header {
   padding: 10px;
   padding-top: 15px;
-  background-color: white;
   padding-left: $header_horizontal_spacing;
   
   .feedback-column-title {