Fix for issue #15841 && #15495 (#16083)

* Centering of <thead> in device summary table

I add the property "text-align: center;" to class ".gridster *"

* Adding lines in app.css

I add these two lines to the style sheet:

.select2-dropdown:is(.tw-dark *) {background-color: #2E3338;}

.select2-results__group:is(.tw-dark *) {border: 1px solid #1f2225;}

html/css/styles.css

@@ -1726,6 +1726,7 @@ tr.search:nth-child(odd) {
 
 .gridster * {
   margin:0;
+  text-align: center;
 }
 
 .grid ul {