[UI] Minor adjustments to styling and classes (#3843)

* Add hint class to Load Average details label
* Remove shadow from table header on flat theme
* Add border to top of table header on /list/server/

web/css/src/themes/flat.css

@@ -57,6 +57,11 @@
 /* Units table
    ========================================================================== */
 
+.units-table-header {
+	box-shadow: none;
+	border-top: 1px solid var(--chart-grid-color);
+}
+
 .units-table-row {
 	@media (--viewport-large) {
 		&:hover {