Container and Text widget settings can be changed in the responsive tab (under advanced settings) for each widget. There is no need for custom CSS code.

By default, you build a website in desktop mode. Once it looks the way you want, switch to tablet and/or mobile mode to see how the site will be displayed on those devices. If you find, for example, that a font is too large to fit the display, go to the font size and reduce it. The original size will still be the default for desktop viewing. If you do not change a setting, it inherits.