Yes, he is correct.
Check your theme first, via Customizer and/or Theme Options menu item in the left side of WordPress Admin.
If the theme does not have enough or the right options to change the font sizing, which you are referring to “responsive font size”, then you need to use Custom CSS found in the Customizer section or “style.css” file found only in Child themes by going here, Appearance > Editor > style.css. (I recommend using the Custom CSS module found in Customizer instead of style.css).
The responsive font units (em, rem, %, vw, vh, vmin, vmax, [and less supported ex, ch]) are used. You can read more about it here,
https://www.w3schools.com/howto/howto_css_responsive_text.asp
https://css-tricks.com/viewport-sized-typography/
https://www.w3schools.com/cssref/pr_font_font-size.asp
Always test and remember these font size units are not the end-all-be-all in themselves. You will find out their limitations and if they work well with your theme through testing.
!important
Is a CSS rule that will over-ride the theme’s default CSS rule for a CSS specific property. You test first, you may not need to use it in the Custom CSS text window, but if you find the Custom CSS property & value you are using does not seem to work, add !important like so,
.cssClassname {
font-size: 1.2em !important;
}
If this does not work, then you need to find the correct CSS class or ID name that does apply to the changes you want to make and that takes investigation using the browsers developer feature by right-clicking the font you want to change in the browser window and click “inspect element” in the right-click context menu.
…and yes, experience with CSS coding is a requirement for this.
Hope that helps.
