As far as I know, CSS is synonym of trial and error mode! 
Said that, something that helps is running the app in its own tab, so you don’t have the IDE bothering you. Then right click on the icon and click on inspect. The dev tools window will open, showing both the html and the css. You can explore what styles are affecting whatever you are looking for. You can enable or disable each rule, you can change their values or you can add new rules.
When you find out the rule that you don’t like, search that in the css file.
And if you are like, the trial and error is a little shorter.
On the top left corner of the Elements tab of the dev tools there is an arrow that allows you to select any element visible in the page (in Chrome in Windows).