As a rule, you should change modules downloaded from the app store. If you do, you’ll have rework / issues if you need to upgrade a module.
Most modules provide snippets of important page elements you’ll need in order to use the module. You can then include snippets on pages you create in one of your modules using the styling you are currently using. Unfortunately, Excel Importer is not one of these. What I do with Excel Importer is copy the TabContainer from the ExcelImportOverview page in the module. Then I create a page in one of my modules (with appropriate theming) and paste that TabContainer onto the page I just created. This new page that I create is the page I use to manage the Excel Importer module.
Hope that helps,
that works like a charm, thanks a lot!