Update test cases for softened duo themes

tests/testthat/css/duo-header_bg.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #627A89;
+  background: #355467;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #627A89;
+  background-color: #355467;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);

tests/testthat/css/duo.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #627A89;
+  background: #355467;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #627A89;
+  background-color: #355467;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);

tests/testthat/css/duo_accent.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #DDD5B3;
+  background: #D8CFA8;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #DDD5B3;
+  background-color: #D8CFA8;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);

tests/testthat/css/duo_accent_inverse.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #004831;
+  background: #005238;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #004831;
+  background-color: #005238;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);