In template and/or its css setting or you create a custom.css file in template root and there you can do all magic everywhere..

https://www.w3schools.com/cssref/pr_font_font-size.asp

you can to that for example for font-size: 15px;

Why is there no change in the CSS defined in the theme word?

https://jmp.sh/mgId5vz

Unfortunately, you can't randomly increase the font size by using a random HTML element because it depends on the current stylesheet where it set the font size.

Can you try to apply the following custom CSS and see how it goes?

body #es,

body #es .es-stream-meta .es-stream-title,

body #es .es-stream-content {

  font-size: 15px;  

}

If still can't manage to increase the font size, you can share with us which page and which part you want to show the big font?