I will go straight forward. I would like to edit let say -> .copyright in my theme. How you are managing to find right place to edit it? I know i could use search in files but that is to long process.
For example i tried: app\design\frontend\Magento\Venustheme\vigoss\web\css\source\_layout_extend.less
but that didn't work to putting out there this class. So how you know where to put css classes? and where they should be overridden?
Use a browser developer tool to find out what CSS classes are already applied to a DOM element (and therefore should be overriden).
I use Google Chrome Developer Tools: https://developers.google.com/web/tools/chrome-devtools/