If you want to change the appearance of the Popup-Window (Feature-Info) in the WebAppBuilder Application, you have to change the esri main style sheet. Normally the PopupWindow gets it's style from the following style sheet:
It's referenced in the following WAB file:
init.js in the folder \server\[yourAppName]
If you download the above mentioned style sheet to you local computer and then link the downloaded ersi.css in the init.js file you can make changes in the PopupWindow Style.
I made the PopUp-Window moveable and hence the pointer in the PopUp-Window weren't very useful anymore. So I changed the esri.css file and it worked great.