If I understand you correctly, you'd like to have an application that you can use to replace the current icons for different applications in the system, in a kind of visual way?

That is probably not trivial since icons in the Jolla device are scattered in different places;
System icons are (mostly, I think) in "/usr/share/themes/jolla-ambient/meegotouch/icons/"
User applications are in subdirectories under "/usr/share/icons/hicolor/"

As for restarting the device, that is not necessary when you change icons, the change is visible just like that after replacing the file.
(well you might need to restart lipstick but it is probably not necessary)