Thank you very much for your reply and your help!
I have set my “Body and Content” and “Headings” in the “Fonts” menu (see attachment), so in the Theme Options, I choose this font presets which works well.
But I have noticed that before loading this fonts (Poppins and Muli), the website shows the system fonts of the device which are Serif-Fonts like Times or so.
Sometimes, the fonts in the Pro-Menu where not loaded at all, which is the font “Montserrat”. Then the menu is displayed with a Serif-Font too (see attachment).
Is there a possibility to set the first choice of the system fonts to “sans-serif” if the webfonts are not loaded?
Thank you very much for your help!