How the custom CSS option works ?

First of all - ALWAYS make a child theme so that updates don't remove any customization you might make. Than you create your own style sheet and your own javascript file. Those are placed in the app-> public -> wp-content folder under another folder named whatever your site is going to be called. You can search the web for the info you need in those custom files so that the child theme will actually or contact me directly and I'll point you in the right direction. Lots of info on the web. All style changes and "tweaks" to javascript functionality are made in your custom files

I honestly do not know. I use a programmer to maintain my site.

It depends on your theme, often in the "Theme Options" panel there is a "Custom CSS" listing where you can add your own custom CSS. Also many themes now have color and style options built right in to reduce the amount of CSS you have to add yourself.

Well it use to be quite easy and they had an editor to change the stylesheets in Wordpress but now they don't really have that option anymore. If you want to work with barebone stylesheets, I recommend downloading your theme and using Dreamweaver or Visual Editor to make changes.