Please kindly read this article to know how to make use of the Font Manager and assign it to different portions of the website:
For the overall theme generated text, you have the options available for you in X > Theme Options > Typography.
If you use the Cornerstone to add the page, then each element has a separate option to set the font. For example, you can learn more about the Text element here:
Now, for the mobile view you have three options to change the font size:
1- You can use the Responsive text tricks and set the font size in a way that it will be responsive. You can learn about that here.
2- You can hide the element that you used in Cornerstone for mobile and add another element and set the font smaller. Please kindly click here for more information on how to hide elements in various viewports.
3- You need to use custom CSS code to force the smaller font size. (This is customization and outside of our support scope.)