Update css reference files for tests

tests/testthat/css/duo-header_bg.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #355467;
+  background: #E8ECEE;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #355467;
+  background-color: #E8ECEE;
 }
 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: #355467;
+  background: #E8ECEE;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #355467;
+  background-color: #E8ECEE;
 }
 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: #D8CFA8;
+  background: #F5F3E9;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #D8CFA8;
+  background-color: #F5F3E9;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);

tests/testthat/css/mono_accent.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #C6C6DB;
+  background: #D9D9E7;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #C6C6DB;
+  background-color: #D9D9E7;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);

tests/testthat/css/mono_accent_inverse.css

   padding: 5px;
 }
 .remark-slide thead, .remark-slide tfoot, .remark-slide tr:nth-child(even) {
-  background: #122D2F;
+  background: #0B1E1F;
 }
 table.dataTable tbody {
   background-color: var(--background-color);
   background-color: var(--background-color);
 }
 table.dataTable.display tbody tr.even {
-  background-color: #122D2F;
+  background-color: #0B1E1F;
 }
 table.dataTable.hover tbody tr:hover, table.dataTable.display tbody tr:hover {
   background-color: rgba(255, 255, 255, 0.5);