theming: properly apply theia-file-icons theme (#12419)

The commit fixes an issue which prevented open tabs from properly displaying the `theia-file-icons` theme resulting in a broken UI.

Signed-off-by: FernandoAscencio <fernando.ascencio.cama@ericsson.com>

packages/core/src/browser/icon-theme-service.ts

@@ -168,9 +168,9 @@ export class IconThemeService {
      */
     setCurrent(newCurrent: IconTheme, persistSetting = true): void {
         if (newCurrent !== this.getCurrent()) {
+            this.activeTheme = newCurrent;
             this.toDeactivate.dispose();
             this.toDeactivate.push(newCurrent.activate());
-            this.activeTheme = newCurrent;
             this.onDidChangeCurrentEmitter.fire(newCurrent.id);
         }
         if (persistSetting) {