fix(tabs): item bakcground issues

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/tabs/_tabs-theme.scss

@@ -62,10 +62,10 @@
         overflow: hidden;
         flex: 0 0 auto;
         z-index: 1;
+        background: var-get($theme, 'item-background');
 
         @if $bootstrap-theme {
             position: relative;
-            background: var-get($theme, 'item-background');
 
             &::after {
                 content: '';