fix(Advanced Filtering): Fix icon colors inside filtering dialog

Author: MPopov

projects/igniteui-angular/src/lib/core/styles/components/button/_button-theme.scss

@@ -785,6 +785,10 @@
         &%igx-button--fab,
         &%igx-button--icon {
             color: --var($theme, 'disabled-color');
+
+            igx-icon {
+                color: inherit;
+            }
         }
 
         &%igx-button--raised,