Can apps from SharePoint store be customized using visual studio or designer

Hi Guys,

In SharePoint 2013, apps or webparts can be downloaded from the SharePoint App store and be utilised in your sites. I wanted to know if these apps can be edited or customized to suit your own sites' look-n-feel. For example, I wanted to showcase a single stock quote in my company's site and found an app called 'Our Stock Price' in the SharePoint Store, but it doesn't quite fit in to my site look-n-feel. So, I thought instead of creating a new App or Webpart for this can I customize this app I downloaded from the App store. I tried googling on this but can't get a comprehensive 'Yes' or 'No'.

Can anyone confirm on this, whether any apps downloaded from app store are customizable??

Thanks Guys

P.S. -- I am working with SharePoint Server 2013 on-premise