change colors of time machine dialog (#5891)

theme/style.less

@@ -79,6 +79,17 @@
     }
 }
 
+.time-machine-header {
+    background-color: @blue;
+}
+
+.time-machine-preview {
+    // Loading div that appears while projects are importing
+    & > div {
+        background: #e8e8e8
+    }
+}
+
 #root:not(.hc) .tutorial-menu {
     .ui.circular.label.selected {
         background-color: @purple !important;