You might want to take a look at this article:
In short it means that you can edit the .scss files in realtime and see the changes in your browser.
Never tried but a quick google gives this as an answer:
right click on the page and press "inspect". That will pull up the developer tools and you can select which element you want to add styling too.
Aware of those, looks like theres a way to edit the scss file in chrome and see the effects in the browser, even apply changes to the local file. I'm close to getting it to work, thought maybe the forum knew how to get it working. Saw a video from Mendix showing the functionality but it doesn't show how to do it.
Press F12 when the app starts in Chrome or right click and press inspect to find the exact object that you want to edit.