How to visually modify the app interface and notes CSS in real time?

For websites I use Google chrome web dev tools (F12), but how can I modify the CSS of the GUI and notes in real time and see my changes live?

Additionally, is there a "restart" button or shortcut, everytime I change a style , need to restart Jopplin

You can launch the Chromium developer tools in Joplin as well. Click Tools, then toggle development tools.