is it possible to use the font size in the same unit? For example every font-size in % or px, because sometimes the font-size is set in px and sometimes it is set in %?
By default for the font-sizes in the theme settings the unit is px, there is no % anywhere in the theme settings. Please Clear the cache and check back.
It’s up to you that you can use font-size in % or in pixels(px), there is no such option available to force this criteria.
I saw the screenshot you attached. Unfortunately, there is no such option in the theme settings to override the same unit at once. But you can override it in the theme style.css file and other files as well by editing those files one by one.