If you want to make change on existing style of an element. You will need to find CSS selector of that element. The best way is to inspect site with browser developer tool. There are few tools which help you archive that.
Chrome Developer Tools
If you use Chrome, or have Chrome available, you already have built in browser tools to help you inspect your site.
Firebug is a browser extension built for Firefox, but also has light versions for Safari, Chrome, and Internet Explorer. You can download it for free from below link.
Add Custom CSS to all pages
You can add global custom CSS code for all pages via Theme Setting > Script and you can put the code into different screen size there.
Add Custom CSS styling to Content Builder module
Some of content builder modules have “Custom CSS” field so you can enter CSS styling for example font-size:14px to overwrite styling of module content.