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/ImageThumbnail.styl

@@ -6,7 +6,7 @@
   --sidebar-transition: all 0.3s ease;
 }
 
-.ThumbnailEntry.active .ImageThumbnail
+.ImageThumbnail.active
   border-color: var(--active-color);
   box-shadow: none
   transition: var(--sidebar-transition);
@@ -14,7 +14,7 @@
 .ImageThumbnail
   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