First thing, where to make a change: Make your main-branch available in Mendix Studio (Version control->manage branch lines: Button ‘enable for Mendix Studio). Go to Mendix Studio and click the pencil at the left buttom. This will give you a basic set of styling options. If that does not suffice then theme\web\custom-variable.scss is the place to make changes to styling. Once the change is made and saved, you need to compile the scss to css. Upto Mx8 this was done using Calypso. Since version 9.x Mendix Studio has incorporated this compilation action. So now, when you have a Mendix project running locally, started from StudioPro, open it in a browser. Now, using your prefered editor, you can make and save a change in the custom.scss and after a couple of seconds, you will you changes reflected in the browser. (Mx 9.4 does it even better)
Second thing, what change do you need to make: first have a look if you can profit from the set of parameters that Mendix offers you in custom-variable.scss. Simply set a value of one of them and save. It should reflect in you browser shortly after.
More info: https://docs.mendix.com/howto/front-end/customize-styling-new