Re #2259: UI refinements for warnings (#2397)

* Re #2259: highlight series thumbnails border for active series (series in the active viewport)
* Re #2259: add the warning icon also on the active viewport as an overlay

platform/ui/src/components/studyBrowser/Thumbnail.styl

@@ -2,8 +2,15 @@
 
 :root {
   --series-count-background-color: #678696;
+  --active-color: #20A5D6;
 }
 
+.thumbnail.active
+  .alt-image-text
+    border-color: var(--active-color);
+    box-shadow: none
+    transition: var(--sidebar-transition);
+
 .thumbnail
   cursor: pointer
   display: table
@@ -16,7 +23,7 @@
     justify-content: center;
     background-color: var(--primary-background-color);
     box-shadow: inset 0 0 0 1px var(--ui-border-color-dark);
-    border: 5px solid transparent;
+    border: 2px solid transparent;
     border-radius: 12px;
     height: 135px;
     margin: 0 auto;